Catalogue of ${>}\,55$ MeV Wide-longitude Solar Proton Events Observed by SOHO, ACE, and the STEREOs at ${\approx}\,1$ AU During 2009 – 2016

Abstract

Based on energetic particle observations made at ${\approx}\,1$ AU, we present a catalogue of 46 wide-longitude (${>}\,45^{\circ}$) solar energetic particle (SEP) events detected at multiple locations during 2009 – 2016. The particle kinetic energies of interest were chosen as ${>}\,55$ MeV for protons and 0.18 – 0.31 MeV for electrons. We make use of proton data from the Solar and Heliospheric Observatory/Energetic and Relativistic Nuclei and Electron Experiment (SOHO/ERNE) and the Solar Terrestrial Relations Observatory/High Energy Telescopes (STEREO/HET), together with electron data from the Advanced Composition Explorer/Electron, Proton, and Alpha Monitor (ACE/EPAM) and the STEREO/Solar Electron and Proton Telescopes (SEPT). We consider soft X-ray data from the Geostationary Operational Environmental Satellites (GOES) and coronal mass ejection (CME) observations made with the SOHO/Large Angle and Spectrometric Coronagraph (LASCO) and STEREO/Coronagraphs 1 and 2 (COR1, COR2) to establish the probable associations between SEP events and the related solar phenomena. Event onset times and peak intensities are determined; velocity dispersion analysis (VDA) and time-shifting analysis (TSA) are performed for protons; TSA is performed for electrons. In our event sample, there is a tendency for the highest peak intensities to occur when the observer is magnetically connected to solar regions west of the flare. Our estimates for the mean event width, derived as the standard deviation of a Gaussian curve modelling the SEP intensities (protons ${\approx}\,44^{\circ}$, electrons ${\approx}\,50^{\circ}$), largely agree with previous results for lower-energy SEPs. SEP release times with respect to event flares, as well as the event rise times, show no simple dependence on the observer’s connection angle, suggesting that the source region extent and dominant particle acceleration and transport mechanisms are important in defining these characteristics of an event. There is no marked difference between the speed distributions of the CMEs related to wide events and the CMEs related to all near-Earth SEP events of similar energy range from the same time period.

Appendix: Notes on Particle Data

A.1 Proton Intensity Intercalibration

Although a detailed attempt to intercalibrate the measured proton intensities accurately between different instruments, so as to account for differences in instrument efficiency, was felt to be outside the scope of this article, an intensity comparison and a simple tentative intercalibration was performed to evaluate the need for correction factors. To ensure that the spacecraft observations were made of nearly identical SEP populations, the decay phase of the high-energy proton event of 14 December 2006 was chosen for this purpose. The time range of interest was taken to span from the noon of 15 December to 20:00 UT on 16 December, eliminating the rise phase and ending approximately when the STEREO-A/HET measured 40 – 100 MeV proton intensity fell below ${\approx}\,2.0\times10^{-3}~\mbox{pfu}\,\mbox{MeV}^{-1}$. To compensate for the imperfect match between the energy bins of HET and ERNE, the intensities recorded by the former were separately examined in two energy ranges, 40 – 100 MeV and 60 – 100 MeV. The background intensities were visually estimated as ${\approx}\,5.0\times10^{-4}~\mbox{pfu} \,\mbox{MeV}^{-1}$ for ERNE, ${\approx}\,1.0\times10^{-4}~\mbox{pfu}\,\mbox{MeV}^{-1}$ for the HET 40 – 60 MeV channels, ${\approx}\,4.0\times10^{-4}~\mbox{pfu}\,\mbox{MeV}^{-1}$ for the HET 60 – 100 MeV channels, and ${\approx}\, 3.0\times10^{-4}~\mbox{pfu}\,\mbox{MeV}^{-1}$ for the combined HET 40 – 100 MeV channels of both STEREO-A and -B; these were subtracted from the measured intensities before the intercalibration analysis. The time resolution was one hour for all data sets. In the following, (the intensities of) the energy channels mentioned above are referred to as $I_{\mathrm{ERNE}}$, $I_{\mathrm{HET} 40\,\text{--}\,100~\text{MeV}}$, $I_{\mathrm{HET} 40\,\text{--}\,60~\text{MeV}}$, and $I_{\mathrm{HET} 60\,\text{--}\,100~\text{MeV}}$; the respective quiet-time background intensities have been subtracted.

The average and the standard deviation of the intensity ratio HET-A/HET-B during the period of interest are 1.09 and 0.09 for the 40 – 60 MeV channels, 0.92 and 0.11 for the 60 – 100 MeV channel, and 1.02 and 0.08 for the 40 – 100 MeV channel; these intensities, with the background intensities given above subtracted, are shown in Figure 21. The results suggest that the proton intensities measured in this energy range by both HETs usually differ by not more than some 20%, possibly less still in the 40 – 100 MeV combined channel, which enjoys better count statistics than its constituent channels during times of elevated particle flux. For this reason, an intercalibration between HET-A and HET-B was not considered necessary. In the following, the HET-A proton intensities are taken as representative of both HET-A and HET-B as regards any comparison with ERNE.

Figure 21

HET-A (blue) and HET-B (red) proton intensities, with quiet-time background intensities subtracted, during the comparison period. From the upper panel down: $I_{\mathrm{HET} 40\,\text{--}\,100~\text{MeV}}$, $I_{\mathrm{HET} 40\,\text{--}\,60~\text{MeV}}$, and $I_{\mathrm{HET} 60\,\text{--}\,100~\text{MeV}}$. The time resolution is one hour.

For the purpose of the intercalibration, a linear combination of $I_{\mathrm{HET} 40\,\text{--}\,60~\text{MeV}}$ and $I_{\mathrm{HET} 60\,\text{--}\,100~\text{MeV}}$ was defined as follows:

$$ I_{\mathrm{HET},\mathrm{lin}} = aI_{\mathrm{HET} 40\,\text{--}\,60~\text{MeV}} + (1-a)I_{\mathrm{HET} 60\,\text{--}\,100~\text{MeV}}, $$

(7)

where $a$ is a parameter within the range $[0,1]$. Choosing $a$ so that $I_{\mathrm{HET},\mathrm{lin}}$ coincides as closely as possible with $I_{\mathrm{ERNE}}$ and then comparing the uncorrected maximum intensities in $I_{\mathrm{HET} 40\,\text{--}\,100~\text{MeV}}$ with $I_{\mathrm{HET},\mathrm{lin}}$ gives a coarse estimate of the measured intensity difference between ERNE and HET and, therefore, of the intercalibration factor. Minimizing the sum of the squares of the quantity $\log_{10}(I_{\mathrm{ERNE}}) - \log_{10}(I_{\mathrm{HET},\mathrm{lin}})$ over the time range of interest suggests that $I_{\mathrm{HET},\mathrm{lin}}$ and $I_{\mathrm{ERNE}}$ coincide best when $a \approx 0.78$. However, it must be emphasized that this result is strictly applicable only to periods of moderate, decreasing proton intensities. The intensities $I_{ \mathrm{HET},\mathrm{lin}}$ and $I_{\mathrm{ERNE}}$, with $a = 0.78$, are shown in Figure 22 for the comparison period.

Figure 22

$I_{\mathrm{ERNE}}$ (red line and crosses) and $I_{\mathrm{HET},\mathrm{lin}}$ (blue line and triangles), when $a = 0.78$ (see text). Time resolution is one hour.

To study the effects of this correction briefly, the peak intensities of the events considered in this work were determined using both $I_{\mathrm{HET},\mathrm{lin}}$ and $I_{\mathrm{HET} 40\,\text{--}\,100~\text{MeV}}$, with $a$ set to 0.78. The maximum intensity values derived from $I_{\mathrm{HET},\mathrm{lin}}$ are between 1.3 and 2.2 times greater than those derived from $I_{\mathrm{HET} 40\,\text{--}\,100~\text{MeV}}$, which implies that the intercalibration factor for the ERNE and HET high-energy proton channels of interest would also lie approximately in this range.

A visual inspection of ERNE and HET data recorded in December 2006 confirms that $I_{\mathrm{ERNE}}$ and $I_{\mathrm{HET} 40\,\text{--}\,100~\text{MeV}}$ typically agree to within a factor of ${\approx}\,2$ during periods of clearly enhanced proton flux and good data coverage. The only notable exceptions to this are the short, very high intensity spike in HET data on 12 December 2006 and a period of about 24 hours during the decay phase of the 13 December 2006 event. ERNE did not detect anything corresponding to the first feature, but in contrast appears to have resolved some structure not readily visible in the 1-hour-averaged HET data in the latter case. Such discrepancies are likely to be due to local structures in the proton flux.

Considering the fact that when widely separated, SOHO and the STEREOs encounter different particle populations, a precise proton intensity intercalibration between ERNE and the HETs would be, in general, not possible. For this reason, it was not pursued here any further and the measured proton intensity values are reported and analysed as such for each observing spacecraft, without applying any correction. In the light of the results presented above, it nevertheless does not seem unreasonable to expect that the measured intensities are likely accurate and comparable to each other within a factor of ${\approx}\,2$ in most cases.

A.2 Electron Data Intercalibration

Approximate intercalibration factors were determined for STEREO/SEPT and ACE/EPAM omnidirectional electron intensities. The decay phases of two small electron events in late May of 2007 – during which STEREO-A and -B were separated from the Earth by ${\approx}\,6^{\circ}$ and ${\approx}\,3^{\circ}$, respectively – were chosen for comparison and the data points selected from the approximate time of intensity maximum onward until the 0.165 – 0.335 MeV electron intensity recorded by SEPT-A fell below $10~\mbox{pfu}\,\mbox{MeV}^{-1}$. Since electron intensities are usually more isotropic during the event decay than the onset and peak phases (see e.g. Dresing et al., 2014), only the decay phases were investigated to minimise the effect of differing instrument responses due to particle flux anisotropies. SEPT intensities were separately examined in two energy ranges, 0.165 – 0.335 MeV and 0.195 – 0.335 MeV, to compensate for the imperfect match between the energy bins of SEPT and EPAM. After visually estimated quiet-time background intensities were subtracted ($2.0~\mbox{pfu}\,\mbox{MeV}^{-1}$ for SEPT-A and -B in both energy ranges, $8.0~\mbox{pfu}\,\mbox{MeV}^{-1}$ for EPAM LEFS60, $20.0~\mbox{pfu}\,\mbox{MeV}^{-1}$ for EPAM DE30), the data were smoothed with 15-minute sliding average.⁶

SEPT-A and SEPT-B were considered first. Since the average difference of their intensities was less than 2% in both energy ranges during the period of interest, no intercalibration between them was regarded as necessary. However, the averages and standard deviations of the SEPT-A/EPAM (LEFS60) and the SEPT-A/EPAM (DE30) intensity ratios were next determined, respectively, as $0.96 \pm 0.10$ and $0.74 \pm 0.12$ for the SEPT 0.165 – 0.335 MeV energy range and as $0.72 \pm 0.10$ and $0.56 \pm 0.10$ for the SEPT 0.195 – 0.335 MeV energy range. Based on these results, the intercalibration factor was selected as 0.84 for EPAM LEFS60 and 0.65 for EPAM DE30, these being the mean values of the averages given above for each EPAM data type. It must be noted that, as in the case of protons, explained in Section 2.1, these factors are fully applicable only when moderate and decreasing electron fluxes are being observed.

Figures 23, 24, and 25 demonstrate the intensity intercalibration process for electron data. Figure 23 shows the data with background intensity subtracted for SEPT-A and SEPT-B in the energy ranges 0.165 – 0.335 MeV (upper panel) and 0.195 – 0.335 MeV (lower panel); Figure 24 shows a comparison between SEPT-A 0.165 – 0.335 MeV intensity with EPAM LEFS60 and DE30 intensity (upper panel) and SEPT-A 0.195 – 0.335 MeV intensity with the same EPAM data (lower panel), all with background intensity subtracted; and finally, Figure 25 displays the same data as Figure 24 but with the intercalibration factors applied to both EPAM data types.

Figure 23

SEPT-A (blue) and SEPT-B (red) electron intensity data with quiet-time background intensity of $2.0~\mbox{pfu}\,\mbox{MeV}^{-1}$ subtracted for the 0.165 – 0.335 MeV (upper panel) and the 0.195 – 0.335 MeV (lower panel) combined energy channels. All data are smoothed with a 15-minute sliding average. The dashed vertical lines mark the limits of the comparison time selection.

Figure 24

SEPT-A electron intensity data, with quiet-time background intensity of $2.0~\mbox{pfu}\,\mbox{MeV}^{-1}$ subtracted, for the 0.165 – 0.335 MeV (upper panel) and the 0.195 – 0.335 MeV (lower panel) combined energy channels (blue), each compared with the 0.18 – 0.31 MeV EPAM LEFS60 (red) and DE30 (black) electron intensity data, with quiet-time background intensities of $8.0~\mbox{pfu}\,\mbox{MeV}^{-1}$ and $20.0~\mbox{pfu}\,\mbox{MeV}^{-1}$, respectively, subtracted. All data are smoothed with a 15-minute sliding average. The dashed vertical lines mark the limits of the comparison time selection.

Figure 25

Same as Figure 24, but with correction factors of 0.84 and 0.65 applied to EPAM LEFS60 and DE30 data, respectively. The dashed vertical lines mark the limits of the comparison time selection.