The Balloon-borne Investigation of Temperature and Speed of Electrons in the corona (BITSE): Mission Description and Preliminary Results

We report on the Balloonborne Investigation of Temperature and Speed of Electrons in the corona (BITSE) mission launched recently to observe the solar corona from about 3 Rs to 15 Rs at four wavelengths (393.5, 405.0, 398.7, and 423.4 nm). The BITSE instrument is an externally occulted single stage coronagraph developed at NASA's Goddard Space Flight Center in collaboration with the Korea Astronomy and Space Science Institute (KASI). BITSE used a polarization camera that provided polarization and total brightness images of size 1024 x 1024 pixels. The Wallops Arc Second Pointing (WASP) system developed at NASA's Wallops Flight Facility (WFF) was used for Sun-pointing. The coronagraph and WASP were mounted on a gondola provided by WFF and launched from the Fort Sumner, New Mexico station of Columbia Scientific Balloon Facility (CSBF) on September 18, 2019. BITSE obtained 17,060 coronal images at a float altitude of about 128,000 feet (39 km) over a period of about 4 hrs. BITSE flight software was based on NASA's core Flight System, which was designed to help develop flight quality software. We used EVTM (Ethernet Via Telemetry) to download science data during operations; all images were stored onboard using flash storage. At the end of the mission, all data were recovered and analyzed. Preliminary analysis shows that BITSE imaged the solar minimum corona with the equatorial streamers on the east and west limbs. The narrow streamers observed by BITSE are in good agreement with the geometric properties obtained by SOHO coronagraphs in the overlapping physical domain. In spite of the small signal-to-noise ratio (about 14) we were able to obtain the temperature and flow speed of the western steamer region in the range 4 to 7 Rs as: For the equatorial streamer on the west limb, we obtained a temperature of 1.0 +/- 0.3 MK and a flow speed of about 260 km/s with a large uncertainty interval.


Introduction
The Balloon-borne Investigation of Temperature and Speed of Electrons in the corona (BITSE) mission's primary instrument is an optical telescope fitted with an external occulter that blocks the direct sunlight so the faint emission from the corona can be observed. Unlike traditional coronagraphs, the BITSE coronagraph (BITSE COR) has a single-stage optics and a polarization detector to obtain both the total and polarized brightness of the solar corona at four narrow passbands in the blue end of the K-coronal spectrum. The BITSE mission is specifically built to demonstrate that the temperature and flow speed of the coronal electrons can be measured by quantifying the change in shape and red shift of the K-corona spectrum when the temperature and speed of the electrons change (Menzel and Pasachoff 1968;Cram 1976;Ichimoto et al. 1996;1997;Takahashi et al. 2000;Reginald and Davila, 2000;Reginald et al. 2003;2011). The temperature and flow speed of the electrons along with the density are key input parameters to models of solar wind acceleration. Traditionally, coronagraphs measure just the electron density, while the BITSE coronagraph utilizes spectral information to determine the electron temperature and flow velocity in addition to density.
Direct measurements of electron temperature have generally been not made beyond ~3 Rs until the domain of in-situ measurements. There is only one temperature value (1.1 ±0.3 MK) at a distance of 2.7 Rs obtained by Fineschi et al. (1998) from the measurement of the electron-scattered Ly-α profiles in the corona. Electron temperature measurements in the 3-8 Rs distance range are necessary to determine the bulk-plasma heating rate in different solar wind regions, as well as the energy partitioning between protons and electrons (see e.g., Cranmer et al. 2010). The energy partitioning is a key diagnostic of turbulence models (e.g., Cranmer 2012; Cranmer & van Ballegooijen 2012) as well as a driver of the stability of helmet streamers (e.g., Endeve et al. 2004). Electron temperature measurements are also important in understanding the sunward conduction of electron thermal energy that determines the solar wind mass flux (Withbroe 1988;Lie-Svendsen et al. 2002). Solar wind speed measurements are also rare in the solar wind acceleration region (see Abbo et al. 2016). Grall et al. (1996) used the Very Long Baseline Array (VLBA) to observe interplanetary scintillation of distant radio sources viewed through the corona to show that the fast wind speed in the polar region is ~600 km/s at a distance of ~6.8 Rs indicating that most of the solar wind acceleration has occurred already. Strachan et al. (2002) combined SOHO mission's Ultraviolet Coronagraph Spectrometer (UVCS) and Large Angle and Spectrometric Coronagraph (LASCO) data to show that there is no measurable O5+ outflow along the axis of an equatorial streamer until after ~3.5 Rs, followed by a rapid increase in velocity to ~90 km/s at 5 Rs in the streamer stalk. Dolei, Spadaro, and Ventura (2015) combined UV spectral and white-light observations to derive a flow speed in the range 40-140 km/s at distances of 2.5-5.0 Rs at the edges of streamers.
Thus, there is a clear need for measurements of electron temperature and flow speed in the solar wind acceleration region, 3-8 solar radii (Rs) for constraining solar wind acceleration and heating. The measurement gap is bracketed by spectroscopic measurements made at distances <3 Rs and the Parker Solar Probe measurements to be at distances >9 Rs. For example, the state-of-the-art solar wind models assume an electron temperature variation to obtain the flow speed.
Measured electron temperature input to the models will make the models more realistic. BITSE demonstrates that the electron temperature and flow speed can be measured, providing critical observational constraints.
The BITSE mission is the culmination of several technological activities in coronagraphy undertaken at NASA's Goddard Space Flight Center (NASA/GSFC) over the past several years (Gopalswamy and Gong 2018): (i) A breadboard version of a single-stage, externally occulted coronagraph known as the Goddard Miniature Coronagraph (GMC) was built and tested in the laboratory and in vacuum tank, and (ii) a commercial polarization camera was tested during the 2017 August 21 total solar eclipse in the United States, that obtained both polarization and total brightness images in four narrow band filters Gopalswamy et al. 2018;Cho et al. 2020). The polarization camera allows us to image the corona at different polarization positions simultaneously, instead of sequentially observing at different polarization positions using a polarization wheel. Eclipses last typically only for a couple of minutes, barely long enough obtain images in all filters. A balloon platform offers a much longer period of observations. Balloon-borne coronagraphs observed at long wavelengths have provided valuable information on the density distribution of the corona (see Koutchmy 1988 for a review of past balloon-borne coronagraphs). The BITSE mission demonstrated both the GMC and the polarization camera for the first time in space.
It must be noted that Cram's (1976) technique has been attempted during total solar eclipses in the past. Ichimoto et al. (1996) obtained coronal spectra in the range 3700 to 4729 Å at two heights (1.1 and 2 Rs) during the 1994 November 3 total solar eclipse, compared them with the expected spectra for a given coronal electron density distribution, and obtained an electron temperature of 1.6 MK in a streamer. They also estimated the bulk flow speed of the electrons as ~80 km/s from the change in the intensity ratios at 3990 and 4218 Å. Takahashi et al. (2000) used the intensity ratios 4230 Å/4100 Å and 4220 Å /4100 Å to show that the streamers were hotter than the coronal holes, although they did not report actual temperatures. Reginald and Davila (2000) and Reginald et al. (2003)  but the signal-to-noise ratio was not enough beyond 1.5 Rs to obtain the physical parameters in the solar wind acceleration region.
In this paper we present an overview of the BITSE mission including the instrument, pointing system, onboard computer, and the balloon launch. We also describe various subsystems: the BITSE electronics box (BEB), BITSE camera, filter wheel, optical bench, coronagraph door, thermal control, and the power distribution unit (PDU). Finally, we present images obtained by BITSE and preliminary results on the temperature and flow speed of electrons.

The Coronagraph
BITSE is a single-instrument mission. The instrument, BITSE COR is an externally occulted single-stage coronagraph that images the solar corona at the blue end of the solar spectrum. Single stage means that the optical system does not have an internal occulter and a Lyot stop employed in traditional multi-stage coronagraphs. The coronagraph images the photospheric light Thomson-scattered by coronal electrons that may have different temperatures and flow speeds in different coronal magnetic structures. The coronagraph consists of an optical assembly, an external occulter, straylight baffles, a heat rejection mirror, a filter wheel, and a polarization camera. The coronagraph has only two flight mechanisms: aperture door (one-time opening) and a filter-wheel rotator. This represents one less mechanism (no polarizer wheel) compared to the traditional coronagraphs. The coronagraph is pointed at the Sun using the GSFC/Wallops Arc-Second Pointer (WASP) system. The polarization camera obtained the images that were stored on board and a fraction of the data were sent in real time to the ground station. mm and has a cone angle of 0.35⁰. A1 aperture has a diameter of 50 mm. The 1-m separation between the A1 aperture and EO is to reduce vignetting. Coronal light is focused by the lens group and imaged at the detector array after passing through the bandpass filter and the field corrector lens.  (Gong, Gopalswamy, and Newmark, 2019). The occulter mask on the face of the detector has a diameter of 2.5 mm that corresponds to an inner cutoff at 3 Rs. Based on simulations, it was found that the diffraction straylight level beyond 3 Rs is not sensitive to the EO cutoff, either at 1.5 Rs or close to 3 Rs. The only case with higher level of diffracted light is when EO blocks just the photosphere (1 Rs). We chose 1.5 Rs because it decreases the vignetting significantly near the specified inner cutoff (3 Rs) without compromising the diffraction suppression. The long separation between EO and the A1 aperture reduces the beam vignetting and increases the signal-to-noise ratio to compensate for the noise caused by the sky brightness at the float altitude of ~128,000 ft (~39 km). Table 2 shows the specifications of optical elements used in the coronagraph. L1-L3 constitute the lens group with front (S1) and back (S2) surfaces having different shapes and radii of curvature. All optical filters are flat. The lenses L1, L2, and the field corrector (FC) have the convex-concave shape. Both surfaces of L3 are convex.

Mechanical design
The BITSE COR mechanical design consists of the following primary components: a modular aluminum structure for the telescope tube, external occulter, aperture door mechanism, lens group assembly, filter wheel, polarization camera, thermal radiator system, heat rejection mirror, base plate, the BITSE electronics box (BEB), and the power distribution unit (PDU) (see Fig. 2). The main coronagraph tube is 1004 mm long with a diameter of 260 mm. The external occulter is located at the front edge of the coronagraph tube. BITSE COR also has a 607 mm long and 343 mm diameter straylight baffle tube to prevent glint from the balloon/gondola or  covers the coronagraph over the base plate from the polarization camera at the backend to the front end of the coronagraph tube. It has three cutouts for the camera radiator, harnesses, and the front baffle. The straylight baffle tube is not covered by the thermal enclosure. The door mechanism is for opening once when the instrument reaches float altitude. The instrument total mass is 116 kg.
The bulkhead assembly consists of a heat rejection mirror (HRM), a lens-group subassembly and a filter wheel (see Fig. 2). HRM reflects the solar energy out through the front aperture and directs it away from the occulter assembly and the front tube's internal surfaces. The lens group consists of 3 lenses and 3 lens cells.
Each lens is aligned and bonded into a cell and each cell is aligned to the adjacent cell and then bolted into its aligned position (see Fig. 2). Each lens is positioned onto to a precision flat surface in each cell for tip/tilt alignment. Precision dowel pins are used to center the lens in the cell then a room-temperature-vulcanizing (RTV) adhesive is injected into the cell to bond the lens to the cell; the pins are removed after the adhesive cures. The filter wheel drive mechanism is supported by a pair of angular contact ball bearings and a piezo-motor is operated in an openloop to position the filter elements into the chief ray. A hard stop is used to reposition the wheel to a home position if motor step counts are lost. The camera is mounted onto the aft section of the assembly and a thermal radiator is used to cool the camera's CCD. The camera enclosure is placed inside a 1-atm pressure vessel because the camera is not vacuum compatible. A base plate (optical bench) supports the instrument assembly, the Sun sensor, and a thermal enclosure. Also mounted on the base plate are the aperture-door controller box and the master reference cube for alignment. The base plate also has a hole providing access to the purge hose that connects to the gas inlet in the bulkhead assembly. The base plate, the BITSE electronics box (BEB), and the power distribution unit (PDU) are mounted on the 10-inch telescopic beam, which is part of the WASP system mounted on the balloon gondola.

Filter Wheel Assembly
The filter wheel is a key component of the passband ratio imaging technique. Fig.   3 shows the filter wheel assembly (FWA) with its three elements: the frame with five slots, power converter, and motion controller. The wheel diameter is 156.4 mm. The filter positions are at a distance of 49 mm from the shaft. The filter slots have a diameter of 51.5 mm. It typically takes 1 s to move and position a filter in the optical path of the coronagraph (the move corresponds to an angular rotation of 72⁰). The motion controller has a piezo motor and an optical encoder with a positioning reliability better than 0.5 mm. A three-point pivot alignment system for each filter and bearings around the filter wheel are used to achieve a tilt reliability of ~ 9 arc min. A thermal/vacuum test of the filter wheel confirmed that it can operate in the temperature range -10⁰C to +40⁰C. The FWA power consumption is 2 W (normal) and 4 W (active).   Table 3 shows the specifications of the four narrowband filters and the broadband filter. The central wavelengths are slightly different from the previously employed wavelengths (e.g., in the recent total solar eclipse set up). The substrate of the filters is Ohara S-BSL7 optical glass (equivalent to BK7) with a diameter of 47 mm and thickness of 2 mm. The filters have an antireflection (AR) coating on the back surface. The filter transmittance is over 90% average in the defined bandwidth. The transmittance drops from 90% of the maximum to 10% within 20% of the bandwidth. The filters have an out-of-band rejection to the level of 1×10 -3 from 300 nm to 1100 nm. The filters are optimized for an incident angle of 8⁰ cone for central wavelength.

Focal Plane Assembly
The    with ports for data (USB3.0 A-type female connector) and power (4-pin circular connector). The circular mirror of diameter 12.7 mm is for alignment purposes.      The camera heater is software controlled due to tighter temperature range (6.9⁰C

BITSE Electronics Box (BEB) is commercial RTD box, centered on an Intel
to 7.1⁰C). The temperature range of all the other zones is 18⁰C to 22⁰C, except for the BEB and PDU base plates, which have a range of -2⁰C to 2⁰C. The operational power has been estimated as 133, 64, and 235 W for the float-altitude, pre-launch, and ascent phases, respectively. as an inset (it is located inside the doghouse at the aft end directly below the radiator shown in green).

Flight Software
The Flight Software (FSW) running on BEB is dedicated for subsystem control, system resource management, observation management, data handling, and automation. The FSW is based on the core Flight System (cFS), which is a platform-and project-independent, reusable software framework along with a set of reusable software applications. FSW development took into consideration of the limited communication speed and the need for automatic operation without ground station contact. FSW development preceded the subsystem readiness using hardware and software simulators at different stages.
The FSW architecture has the following three layers: the operating system (OS),  Fig. 9 shows how various applications access BITSE hardware and also WASP and SIP (CSBF). represent applications developed specifically for the BITSE mission.

Ground Support Equipment
Ground Support Equipment (GSE) consists of a science computer, the WFF/WASP computer, and the CSBF system, as well as the line-of-sight (LOS) command transmitter, the LOS Mini-SIP receiver, the LOS 1 Mbps receiver, and the LOS video receiver for WASP (see Fig. 10). The ground computer sends commands to the flight computer via the CSBF command transmitter. Upon execution of the commands, the data are sent down to be accessed by the ground science computer.
Science data are downloaded using an 8 Mbps EVTM downlink.

BITSE Integration and Launch
The filter wheel, camera, and BEB manufactured and tested at KASI, then received at GSFC and integrated with the optical and mechanical components in a clean tent. After alignment, the coronagraph was shipped to WFF where it was integrated with the gondola that provides the pointing mechanism needed for BITSE observations. Environmental tests have been performed at various levels from the subsystems to end-to-end. The Wallops Arc Second Pointing (WASP) system (http://sites.wff.nasa.gov/balloons/technology_wasp_details.html) provides <1 arc sec pointing accuracy and sub arcsecond pointing stability, much more than required by BITSE. The pointing is achieved by a rectangular, hollow telescope beam mounted on a gimbal system controlled by two opposing gimbal hubs per axis (pitch and yaw). The telescope beam is sturdy enough to provide structural stability during flight and absorb any significant forces during landing.
The gondola and the WASP system have been successfully used in previous test and science flights. The WASP system, the WASP avionics deck, the CSBF battery packs, and the science stack are accommodated on the gondola. The WASP avionics consists of a ~21" × ~17" (~53 cm × ~43 cm) aluminum deck, the main flight computer, enclosures for resolver interface, power relays, motor driver interface, GPS receiver, H-Bridge circuits, and housekeeping.
The integration involved the following major steps: (i) attach the coronagraph on the telescope beam of the gondola such that the front baffle extends to the front of the beam, (ii) place and secure the BEB and PDU boxes behind the coronagraph on the telescope beam with base plates, (iii) complete the harnessing, (iv) fasten the WFF Sun sensor to the side of the optical bench. Figure 12 shows the coronagraph integrated to the gondola and the WASP system. The gondola with all CSBF hardware had a total mass of ~3200 lb (~1450 kg).
Adding the science payload mass, secondary payloads, steel ballast (~900 lb or ~408 kg), and the parachute the total mass became ~5500 lb (~2500 kg). Given the fact that the atmospheric scattering of sunlight is severe at the blue end, we chose the balloon that attains the highest possible altitude. Therefore, we chose the 39 million cubic feet (1.1 million cubic meter) balloon that attains a float altitude of ~130,000 feet (~40 km). The mission was ready for launch in early   Table 4 lists the number of images obtained. Seventy frames were obtained in each narrow-band filter and two frames in the broadband filter per observing set. There were 61 sets in Filter 1 (F1) and F2 and only 60 sets in F3, F4 and F5, yielding 17,060 coronal images over an observing period of ~4 hours.
The cadence of the sets is ~3.5 min. For dark images, 40 frames were obtained in each filter per set. Two dark sets were obtained at flight altitude, yielding 400 dark images.  displayed is the ratio of the total brightness in set 7 to that in set 6. In each set, there are seventy images, which are added together.   The FOV also contains a ghost feature and scattered light in the instrument. A major contribution to the background is the sky brightness, which is not completely eliminated at the float altitude. Figure 15 shows snapshots of the coronagraph and its surroundings taken by the onboard video camera. Even though the sky looks dark at float altitude, there is still significant sky brightness level. The diffraction stray light also equally contributes to the background. The main problem in coronagraphic images is subtracting the background. Since we need to measure the polarization brightness to isolate the K-corona, we make use of the field rotation to isolate the signal from the streamer.

Background Subtraction and Polarized Brightness
The 1024 ×1024 images are rebinned to 256 × 256 for further processing. In this process, hot pixels are removed in each of the new macropixels. We subtract the background from polarized brightness (pB) image as follows. We track the stokes variables S1 = I0 -I90 and S2 = I45 -I135 as a function of time in each pixel in the 256 × 256 images. The pixels that are crossed by the streamer (e.g., the pixel denoted by a diamond in Fig. 16a) show an enhancement during the streamer transit (due to FOV rotation). Before and after the streamer transit, the intensity in these pixels are close to the background, similar to the pixel denoted by a square symbol in Fig.16a. Repeating this procedure for every pixel in the image, we get the pB image as pB = (S1 2 + S2 2 ) 1/2 .
(1) Figures 16b shows I0, I45, I90, and I135 at the diamond pixel in Fig. 16a. Because the sky brightness increased due to the decreasing balloon altitude, all intensities increased with time. It is difficult to see the streamer signals in Fig. 16b because the background (sky and the stray light) was not constant. However, the streamer signals are clearly seen in the S1 signal (= I0 -I90) as shown in Fig. 16d as the change of the background S1 signal was relatively small.
The streamer signal is not seen in S2 (= I45 -I135) but this is expected because the signal is vertically polarized at the streamer PA=270⁰ (polarization angle is tangential). Similarly, Stokes S2 = I45 -I135 is shown in (e) along with the third order polynomial fit to the background data points. (f) S1 and S2 after subtracting the fitted background (denoted with subscripts BKGSUB). S1 and S2 change with time. When the fitted background from (d) and (e) are subtracted, the background values of S1 and S2 are at zero with small fluctuations. The procedure is repeated for the "square" pixel in (a) as shown in (g), (h), (i).
To estimate the background values of S1 and S2 signals during the streamer transit, we fit third-order polynomials to the off-streamer data (plus symbols) and subtract the fitted values from the S1 and S2 signals as shown in Figure 16f. Figure 16c shows the variation of I0, I45, I90, and I135 at the square pixel in Fig. 16a. The western streamer did not transit this pixel during the observation. Figures 16g and   16h show the S1 and S2 signals of the square pixel along with the third-order polynomial fits. When the fitted values are subtracted from S1 and S2 we see the fluctuation of the background around zero (Figure 16i).
We can use Fig. 16f to estimate the signal-to-noise ratio in the diamond pixel. The standard deviation of the off-streamer points (plus symbols in Fig. 16f) is 4.20 DN on S1 and 3.99 DN on S2. A similar noise level is expected in the on-streamer points (circles in Fig. 16f). Using the average S1 (-14.93 DN) and S2  and the corresponding noise levels, we get the SNR in the pB signal as 2.6 according to eq. (1). The SNR is low, but it corresponds to a snapshot image. We need to do temporal and spatial averaging of the images to improve the SNR.
improved by rebinning the 256×256 image to a size of 32×32 pixels. The background subtraction method outlined above is applied to all the pixels in the images taken at the four filters. After co-adding 30 sets of images, we obtain the pB images shown in Fig. 17. BITSE was launched at the end of solar cycle 24, so the corona has a simple structure with equatorial streamers above both limbs (marked 2 and 3 in Fig. 17). There was an additional thin streamer at the location marked 1, but this streamer is not discernible in the BITSE image. A closer look at streamer 2 indicates that there are striations in the lateral direction, which is likely to be due to insufficient flat-fielding. The offset between the occulter image (BITSE pointing center) and the optical disk was introduced in the beginning of the observations to make the features look symmetric. The offset is estimated as ~0.63 Rs from the relative positions of the Sun, Zavijava and υ-Leo (see Fig. 14).
This makes the western streamer observed well from 3 Rs onwards, while the eastern streamer correspondingly was observed only from ~4 Rs. For these reasons, we primarily use the western streamer in determining the temperature and flow speed of electrons.  Streamer 1 is clear in the LASCO image and profile, but barely noticeable in the BITSE profile. The BITSE profile is approximately calibrated making use of the fact that LASCO/C2 image was taken with an orange filter and our wavelengths are in the blue end of the photospheric spectrum and assuming that the K-coronal spectrum has similar wavelength variation. Figure 19 shows the PA profile of streamers 2 and 3 in all four filters at 5 Rs compared with the LASCO/C2 profile. Also shown is the radial profile at PA = 275⁰. The LASCO/C2 pB intensity (in unit of mean solar brightness, MSB) is high because it is through a passband at longer wavelengths (orange). Taking account of the relative locations of the four BITSE passbands and the LASCO/C2

Comparison with LASCO images
passband (orange filter) on the photospheric spectra, we estimated the expected intensity at BITSE wavelengths. BITSE/F4 intensity is smaller by a factor of 0.937 than the LASCO/C2 intensity based on the photospheric spectrum. The intensity in the shortest BITSE wavelength is smaller by a factor of 0.520. The intensities in the other two wavelengths fall between these two limits. The signal becomes very weak beyond ~7 Rs in the western streamer as indicated by the fluctuations in the BITSE radial profiles. We also determined LASCO/C2 pB profiles using the solarSoft pB_inverter routine. This profile is in agreement with the LASCO/C2 profile in Fig. 19c, even though the routine uses F-corona subtraction using the model by Koutchmy and Lamy (1985).

Temperature and Speed of Coronal Electrons from Spectral fit
The technique to obtain coronal temperature (T) and speed (V) involves quantifying the shape change and red shift of the K-coronal spectrum (see e.g., Ichimoto et al. 1996). Since we have four data points on the spectrum, we can compare the observed spectrum against the theoretical spectrum to see to what T and V values give the best match between the two spectra. The computation of the theoretical K-corona spectrum requires T, V, and density (n) of electrons along the line-of-sight. Assuming T and V to be constant along the line of sight, we model the density distribution of the streamer using a density multiplier A given (2) where P0 is the peak density enhancement of the streamer above non-streamer region, P1 is the central streamer location (longitude φ = P1), and P2 is the streamer width in longitude. For a spherically symmetric corona (no streamer), P0=0 and A = 1. We computed P0 as follows. We plotted the pB as a function of position angle at a heliocentric distance of 2.5 Rs (not shown). We determined the background level outside the streamers as 8×10 -11 MSB. The western streamer had a peak value of ~1.6×10 -9 MSB, yielding P0 ~20. We assume the same enhancement at different distances. The eastern streamer has a slightly higher enhancement factor (~25), while the small streamer 1 has an enhancement factor of ~5. P1 and P2 are obtained from the time profiles of the polarized brightness in the LASCO/C2 synoptic maps (see Fig. 20). By fitting a Gaussian to these profiles, we get P1 and P2 as shown on the plots. The western streamer had a larger longitudinal extent. At the time of BITSE observation, the western streamer was on the west limb (only 0.26⁰ below the sky plane). The eastern streamer was slightly behind the limb (~4.7⁰), again not too far from the sky plane. The expected K-corona spectrum can be determined as described in Cram (1976) with the modification to include speed (Reginald and Davila, 2000). We use the input photospheric spectrum obtained on the day of BITSE observation from the Solar Irradiance and Climate Explorer (SORCE, Anderson and Cahalan, 2005) data available at http://lasp.colorado.edu/lisird/data/sorce_ssi_l3/. The spectrum is corrected for center-to-limb variation using known limb-darkening functions.
There are several limb darkening models (Cram 1976;Neckel and Labs 1994), but we use the Allen (1973) because different models do not seem to have significant effect on the K-corona spectrum. The coronal density distribution is taken to follow the Baumbach-Allen density model (Allen, 1947) multiplied by the factor A from equation (2). The Thomson-scattered radiation in the radial (Ir) and tangential (It) direction are then computed at each wavelength integrated along the line of sight. These are then combined to get the polarization and total brightness as (It -Ir) and (It + Ir), respectively. The resulting pB values at each wavelength is convolved with the CCD quantum efficiency and the filter profile. We also assume that the transmittance of the lenses and MPA are independent of the wavelengths over the narrow spectral range. Figure 21 shows the computed spectrum for a heliocentric distance of 5 Rs for various temperatures and assuming the flow speed to be zero. We used the QE and filter profiles provided by the manufacturers. The final expected spectrum we use has four data points corresponding to the four filter wavelengths. The only remaining information required for calculations is the atmospheric extinction because some light is lost due to scattering and absorption in the atmosphere above the float altitude. To correct for this, we first used the Simple Another estimate of the atmospheric extinction is obtained making use of the observations of the star Zavijava within the BITSE FOV (see Fig. 14). We compared the intensity of Zavijava observed by BITSE at each filter wavelength with the spectrophotometric data (published by Kiehling, 1987; see also Glushneva et al. 1992) convolved with the filter profiles and the CCD quantum efficiency. The spectrophotometric data was listed at 5 nm resolution, so we linearly interpolated between the data points to match the higher resolution of the BITSE filter profiles. By comparing the observed and expected Zavijava spectrum, we get F2 and F3 wavelength transmittances with respect to that in the F4 wavelength as 1.0006 and 0.9837, respectively. At wavelengths < 400 nm, the observed Zavijava flux has large error (0.05 magnitude), so we assume the attenuation factor at the wavelength of F1 is the same as that of the nearby F3 wavelength. Therefore, to account for the spectral changes due to the atmospheric extinction, F1 and F3 intensities are divided by 0.9837. Spectral fitting using SMARTS and Zavijava extinction corrections yielded similar results, except that the former had a better minimum χ 2 . Therefore, we present the spectral fitting results using the SMARTS extinction. Figure 22 compares the observed (black lines) and expected K-corona spectra (red lines) for different combinations of T and V at a macro pixel (32×32 original pixels) only in the western streamer located at a heliocentric distance of 4.2 Rs.
The expected spectra shown in Fig. 21 are in physical units. To compare with the observed intensity in DN/s, we used the conversion factor as another free parameter. We used a least-squares method to find the best values of T and V for which the observed and expected spectra match as indicated by minimum χ 2 (χ 2 min) the goodness of the fit measure we used in fitting. T was varied between 0.2 and 6 MK, while V is varied between 0 and 800 km/s to find the best T and V corresponding χ 2 min. In Fig. 22a-c, we see that the observed and expected spectra do not match for any speed when T = 0.7 MK. In Fig. 22e, the two spectra match for T =1.0 MK and V =260 km/s. The two spectra deviate noticeably at lower   (Table 3). A complete range of T and V parameters is given in the movies. The error bars in the observed spectrum are formal errors of rebinning the original 1024×1024 image to 32×32 size. Using the least squares method to minimize χ 2 , the best combination of T and V to represent the observation are found to be 1.0 MK and 260 km/s, respectively as seen in (e). Figure 23 shows the temperature and speed values obtained using the method described in Fig. 22 at other pixels that have enough signal. Clearly, the signal is not strong enough on the eastern streamer to obtain reasonable T and V values.
The western streamer has 5 pixels (19)(20)(21)(22)(23)16). These pixels are marked in Fig.   23a by white boxes located approximately at heliocentric distances 3.3 to 6.9 Rs, in steps of 0.9 Rs. The innermost pixel (~3.3 Rs) is bright in Fig. 23a, but it partly overlaps with the region of saturation. The signal is weak in the last two pixels located at 6.0 and 6.9 Rs. The spectral fitting method converges with χ 2 min = 0.009 and 5.022 in the 4.2 Rs (20, 16) and 5.1 Rs (21, 16) pixels. The corresponding best T and V values in these two pixels are (1.0 MK, 260 km/s) and (1.0 MK, 290 km/s). The confidence intervals are estimated by varying the parameters around the optimal values corresponding χ 2 min (see e.g., Bevington and Robinson 2003). The 95% confidence intervals of T are determined from χ 2 min + 4 levels as (0.7 -1.4 MK) and (0.7 -1.3 MK) for these two pixels. In the last two pixels, the best T values are 1.1 ± 0.3 MK (6.0 Rs) and 0.9 ± 0.2 MK (6.9 Rs). If we fix the speed in the last two pixels to be 260 km/s as in the 4.2 Rs pixel the T values remain the same. The speed range is very wide, between 0 and 800 km/s in the last four pixels. We can say that the average temperature in the pixels between 4.2 and 6.9 Rs is 1.0 ± 0.3 MK. The flow speed in the height range is 260 km/s, but the uncertainty is large (0 -800 km/s). Given the fact fast wind is not present in streamers, we can conclude that 260 km/s is quite reasonable.  Fig. 22. (b) and (c) have been rebinned to a size of 32×32 pixels. The marked pixels in (a) correspond to the row pixel 16 and column pixels 19-23. The spectral fitting was performed in each pixel of the western and eastern streamers to determine the best temperature and speed as displayed in (b) and (c). The red and blue pixels give extreme values, which are unphysical because the fitting did not converge. When we assume that these pixels have the speed of the adjacent pixel with good fit, the derived temperature is the same as in (b). The T values in pixels 19-23: 1.5, 1.0, 1.0, 1.1 and 0.9 MK.
The best speeds are 260 and 290 km/s in pixels at 4.2 and 5.1 Rs, respectively.
However, fixing V=260 km/s, the T values remain the same at χ 2 min.  Figure 24 shows plots of the temperature (F2/F1) and speed (F4/F3) ratios as a function of T for three different speeds. The ratios were obtained using the expected pB computed as described in the beginning of section 9.3. Recall that the pixel (20,16) at ~4 Rs on the western streamer has T = 1.0 MK obtained from the spectral fitting method. The temperature corresponds to a T ratio of 1.246 from the expected intensities at filters F2 and F1. The ratio falls smoothly as T increases. For example, as T increases from 1 to 2 MK, the T ratio falls from 1.246 to 1.169 for V=260 km/s. The drop in the T ratio by 7.7% per 1 MK, with a slightly faster rate near 1 MK than near 2 MK. The V ratio rises from 1.174 for 0 km/s to 1.185 for 300 km/s keeping T = 1.0 MK. The rise is by 0.40% for an increase of 100 km/s. Rs. Note that the region <4 Rs is in the saturation region. Figure 25 shows the filter ratio maps (F2/F1) and (F4/F3) along with the T and V maps derived from them. The F2/F1 map in Fig. 25a is much smoother than the F4/F3 map. This is especially true for the western streamer. The distribution of the ratios between 4 and 7 Rs in the western streamer gives an average T ratio of 1.25±0.025, which corresponds to T = 1.1 MK with a confidence interval of 0.9 MK to 1.3 MK. If we restrict the T ratio to the range shown in Fig. 24 (red line, 1.08 to 1.42), then the distribution has an average 1.249 ± 0.010. From Fig. 24, we see that this ratio corresponds to 1.0 ± 0.1 MK, in agreement with the spectral fitting method (see Fig. 23b). This value is an average over the radial distance 4 -7 Rs. We can also convert the T ratio to temperature according to the curve in Fig.   24 in individual pixels. A histogram of the T values in the converted pixels has a slightly higher value: 1.1 ± 0.1 MK. In the eastern streamer there are some clusters of pixels having this temperature, but the signal-to-noise ratio is not good enough to obtain the temperature. The V ratio distribution is too broad to yield V values in the 256×256 image (Fig. 25d). Clearly, the maps are too noisy to infer the speeds. If we restrict the V the ratios to the range indicated by the blue lines in Fig. 24 and convert them to speeds, we get a distribution with an average value 297±66 km/s. Further spatial averaging (rebinning the pB image to a size of 32×32 pixels, obtaining the ratio maps, and converting them to T and V maps) shows that the temperature of the western streamer region to be at 1.0 MK in the four pixels at 4.2 -6.9 Rs (see Fig. 25e) in agreement with what was obtained from spectral fitting (see Fig. 23b). The V values in these pixels are 270 km/s, 650 km/s, 210 km/s and 10 km/s (see Fig. 25f). Clearly 5.1 Rs and 6.9 Rs pixels have extreme values, which are unphysical. In Fig. 25f, there are two pixels with speeds similar to those from spectral fitting: the pixels at 4.2 and 6.0 Rs. The pixel at 5.1 Rs had a speed of 290 km/s from spectral fitting, but in the ratio image, this pixel has a much higher speed (650 km/s bracketed by 270 km/s on the left and 210 km/s on the right). The speed of 650 km/s corresponds to a V ratio of 1.200, which is only 2.6% higher than the ratio for zero speed (1.174). In the future work, we will improve the data analysis with refined calibrations and adding the remaining images.

Discussion
The objective of the BITSE mission was to fly a new single-stage coronagraph to image the corona in four narrowband filters in the blue end of the solar spectrum and to derive the physical parameters in the outer corona (3-8 Rs). The BITSE mission demonstrated that the coronagraph was able to obtain polarized brightness images using the polarization camera. The electron temperature and flow speed were derived from the pB images. The intensity of the K-corona dominates only at heliocentric distances <2 Rs. Beyond 2 Rs, the F-corona is brighter than the Kcorona. For example, at a distance of ~3 Rs, the F-corona is brighter by a factor of ~3. Fortunately, the F-corona is unpolarized up to ~5-6 Rs corresponding to the inner part of the BITSE FOV. Thus, extracting the polarized brightness provides a true measure of the K-corona.
Due to the limited signal-to-noise ratio, we were able to determine the temperature and flow speed on the western streamer. We found that the electron temperature is ~1.0 MK and the flow speed is ~260 km/s over the distance range of 4-7 Rs. We used spectral fitting and passband ratio imaging. Both methods give similar results for temperature, but the speeds have large uncertainties because the speed is extremely sensitive to the filter ratio. Dolei, Spadaro, and Ventura (2015) studied the properties of an equatorial streamer during Solar Cycle 23 minimum by combining visible light and ultraviolet observations. From the electron density derived from LASCO pB images, these authors estimated an electron temperatures in the range 0.6-0.9 MK at 4 Rs and 0.4-0.7 MK at 5 Rs. Their 4 Rs temperature overlaps with our range 0.7 -1.4 MK; the 5 Rs temperatures barely overlap with the BITSE range (0.7 -1.3 MK). On the other hand, electron flow speed is slightly larger than that derived from UVCS observations. It must be noted that the macropixels over which we determined the speed and temperature covers the whole streamer (see Fig. 23a). Therefore, we are unable to differentiate the speeds at the axis and edge of streamers.
The solar minimum streamer has a rather small spatial extent (~10⁰ in position angle). In the longitudinal direction, the western streamer lasts for ~6 days. The same streamer structure was studied by Morgan and Cook (2020) two rotations after the BITSE observations. Their compilation of streamer densities at 5 Rs is in the range (5 -11)×10 4 cm -3 . We have not yet determined the electron density from the BITSE data yet, but we inverted the calibrated LASCO/C2 pB image (see Fig. 19) to get a density of ~7.6×10 4 cm -3 , which is within the above density range. After complete calibration, we plan to obtain the density from BITSE images. Assuming a constant mass flux and using the constraints on proton flux measured by Parker Solar Probe, these authors estimated an outflow in the streamers of 50-120 kms -1 at 4Rs, and 90-250 kms -1 at 8 Rs. Our speeds are slightly larger, but consistent with these estimates, given the large uncertainty in our speed measurements.

Conclusion and Future work
The BITSE mission has demonstrated that measuring the polarized brightness of the corona at several wavelengths and characterizing the shape of the corona is a viable technique to determine the physical properties of the corona in the solar wind acceleration region. Previous attempts to use this technique during solar eclipses sampled the corona much closer to the Sun. BITSE was able to obtain the temperature and speed in the range 4-7 Rs. indirectly. These results are preliminary, and we expect to refine them by (i) adding more images that need to be processed taking account of the slightly different BITSE altitudes, and (ii) completing post-launch calibrations.
One of the technologies we demonstrated is the use of the polarization camera used for the first time in space. Obtaining the polarized brightness using the polarization camera is advantageous over the traditional polarization mechanism that combines observations from three polarization positions. This is also important for future space missions because the polarization camera reduces one mechanism (polarization wheel). We were not able to test the tapered-cone external occulter because the occulter size manufactured was different from the design size. An older design value was used in the manufacture. We ended up using a rings attached to the occulter to bring it to the required size. We expect to use the tapered cone occulter in a follow-up BITSE flight. The follow-up flight will also overcome the shorter-than-planned duration of observation: the late start (due to wind conditions) and early end (due to failure of the WASP system).
BITSE is the first joint solar mission between NASA and KASI, a step to increase technology readiness level toward a next-generation coronagraph (Cho et al., 2017). Through BITSE, NASA and KASI coronagraph teams demonstrated the technology successfully and is developing the Coronal Experiment COronal Diagnostic Experiment (CODEX) that will be flown on the International Space Station 2023. CODEX will overcome the sky brightness and the short observing duration that resulted in insufficient signal-to-noise ratio beyond ~7 Rs.