Solar Protons in the Earth’s Magnetosphere According to Riometric and Satellite Data During the Magnetic Storms of October 2003
Fluxes and boundaries of penetration AQ1 of solar cosmic rays recorded by the CORONAS-F satellite during the super-storms of October 2003 are compared with the measurements of riometric absorption by the world-wide riometer network. Dynamics of the polar cap boundaries is studied at different phases of the magnetic storms. Behavior of the absorption value depending on time of the day and solar protons spectrum is calculated for different phases of the solar cosmic rays flare.
KeywordsMagnetic Storm Solar Proton Proton Flux Penetration Boundary Riometric Absorption
Fluxes and boundaries of penetration of solar cosmic rays recorded by the CORONAS-F satellite during the super-storms of October 2003 are compared with the measurements of riometric absorption by the world-wide riometer network. Dynamics of the polar cap boundaries is studied at different phases of the magnetic storms. Behavior of the absorption value depending on time of the day and solar protons spectrum is calculated for different phases of the solar cosmic rays flare.
Solar cosmic rays (SCR) with energies from few up to hundreds MeV generated during the chromospheric solar flares relatively free penetrate into the tail of the Earth’s magnetosphere, which in ionosphere is projected to the polar cap. Solar protons essentially increase the ionization level in the D-layer of the ionosphere and it leads to the increase of radiowaves absorption within the frequency range of 10–50 MHz. This type of absorption is named a polar cap absorption (PCA), in distinction from analogous effects caused by auroral electrons [1, 2, 3, 4].
Riometer continuously monitoring the level of space radionoise absorption is the most widespread instrument. Although riometric absorption does not carry any information about proton spectrum variations and often is “mixed” with absorption caused by auroral electron precipitation and radio-bursts both of natural (the Sun) and of anthropogenic origin, presence of a wide network of riometers allows to use its data in order to study time variations of SCR both separately and combined with the data of low-altitude satellites.
Previous comparison of the integral flux of solar protons and PCA has shown that proton flux is proportional to the absorption (dB) squared and that the basic input into the absorption is made by the protons with energies over 10–15 MeV [5, 6]. As SCR spectra vary both for different events and during each SCR flare and particles detectors characteristics are different for different satellites, the results of the comparison also differ essentially. Therefore for the quantitative specification it is necessary to conduct individual comparisons for a specific satellite.
The results of the comparison of PCA with solar protons measurements onboard the CORONAS-F satellite during a series of SCR flares and accompanying them extreme geomagnetic activity in October 2003 are reported in the present chapter. The main purpose of this work is to determine the coupling coefficients for the specific channels of the particles detectors onboard the CORONAS-F satellite and to find what proton energy ranges are the most effective concerning ionization of D-layer of the ionosphere. Besides, dynamics of proton flux and motion of the boundary of protons penetration into the Earth’s magnetosphere can be monitored by satellite data and by longitudinal chains of riometers; comparison of these methods is of particular interest.
The period of the increased SCR intensity started on October 26 and lasted through November 6, 2003. Series of October storms and the processes on the Sun and in the interplanetary space, which caused them, were analyzed in detail in collective surveys by collaborations of Russian scientists [7, 8] and separate articles published in special issues of “Cosmic Research.” The results of the analysis of time profile of SCR protons in three energy channels for the period of 26.10.2003–01.11.2003 are presented in Fig. 1.
The orbit of the CORONAS-F satellite is almost circular with inclination of 82.5∘ and altitude of 415–445 km. Detectors whose data was used in the present analysis are described in  and in the chapter “Scientific Set of Instruments \(\ll \) Solar Cosmic Rays ≫ ” of the present book. The analysis is based on riometric data of longitudinal chain of Scandinavia: Abisko (International code ABK; corrected geomagnetic latitude 65.3∘), Hornsund (HOR; 73.8∘), Ivalo (IVA; 65.0∘), Sodankyla (SOD;63.9∘), Rovaniemi (ROV; 63.2∘), Oulu (OUL; 61.5∘) and Juviskyla (JYV; 58.8∘) and Tiksi observatory (TIK; 65.65∘) of the Yakutsk meridian. The frequency of riometers receiver is 30–32 MHz (Sodankyla—25 and 40 MHz). Location of riometric stations of Scandinavian chain and several trajectories of the CORONAS-F satellite on October 30 in magnetic coordination system is presented in Fig. 2. It is seen that the satellite’s trajectories are associated with location of the stations of Scandinavian sector and it is just eligible to compare data. Summary graph of riometric absorption at the Tiksi (Yakutsk network) and Sodankyla (Scandinavia) stations is presented in Fig. 3. Absorption variations indicate both real time dependence of proton flux and daily wave of absorption with maximum at day time (local noon in Sodankyla—at 10:30 UT, in Tiksi—at 03:40 UT).
As far as during the analyzed period the level of substorm activity was very high in order to analyze PCA variations it is necessary to separate input from auroral electrons precipitation (auroral absorption—AA) into total absorption. There is no standard procedure for it. The authors used morphological differences between PCA and AA: the latter are short and at night side coincide with bay-form disturbances of magnetic field. Difference of the morphological characteristics of these two types of absorption is seen in Fig. 4, where absorption registergrams of 30.10.2003 according to the Finnish stations chain are presented. At the night side auroral bays of absorption predominate, at the morning and the day sides—PCAs with marked variation of absorption depending on sun lighting. At daytime it is more difficult to separate AA because absorption bays are more smooth, they last up to several hours and are delayed in time relatively substorm activations at the night sector. Although additional selection criteria, i.e. time dependence of solar protons intensity recorded by the CORONAS-F satellite helps, nevertheless at separate moments identification of short-term variations source stays uncertain, for instance, during the main phase of the October 29 storm.
Relation of PCA to the Solar Protons Flux
Figure 5 presents the results of the comparison of riometric absorption and proton flux in the polar cap for the day and night absorption by our data. Accuracy of PCA’s value calculation within the range of up to 1 dB is determined by the accuracy of the determination of diurnal variation of quiet level of radio-noise and can be less than 0.1 dB. But the level of noise increased during the last decades and complication of the account of electron precipitation into absorption result in a more essential dispersion of points. Taking into account the accuracy and possible mistakes in absorption calculations we’ll note good coincidence of measurements made by two chains and proton flux measurements onboard the satellite. Dashed lines on the graphs are described with expression (1), coefficients K for the day and night absorption and proton energy of 1–5 MeV are equal to 400 and 2000, and for the energy range of 14–26 MeV are equal to 10 and 100, respectively.
In most papers it is supposed that the main input into absorption is made by protons with energies over 10–15 MeV; therefore, good relation of PCA and proton flux of 1–5 MeV energy is rather unexpected. This result is not a simple consequence of synchronous variations of proton flux in general for the spectrum as a whole. Figure 1 shows that correspondence of count rate in different energy channels changes, and for several intervals time variations diverge. For instance, Fig. 6 shows that for the time interval of 08–12 UT of October 30 absorption increase relates to the increase of proton flux within the energy range of 1–5 MeV, because for the rest channels flux does not increase. On October 29 during the period of 02–07 UT the same situation was observed. Character of proton variations diverges again: energy proton flux decreases, and low-energy proton flux increases. By riometer data absorption for these sections also increases, and it proves essential input of protons with energies of 1–5 MeV into absorption.
It should be noted that numeric dependence of the night PCA from solar proton flux was obtained for the first time due to extremely high intensity of the analyzed SCR flare. Among the earlier papers there is only one message about approximately five-fold decrease of absorption at the night sector comparing with the day one . Above-mentioned dependencies show that on the average this ratio reaches 3.7 at the equal proton flux with energies of 14–26 MeV and reaches 2.3 at the equal proton flux with energies of 1–5 MeV. Difference of the ratio value is apparently related to the change of the shape of proton energy spectrum.
By means of riometric data it is possible to recover the time variation of proton flux only for separate sectors of day and nighttime, absorption at intermediate intervals is controlled by the angle of the Sun’s elevation and other factors, which can be hardly taken into account. It is also necessary to dismiss the intervals of strong auroral activity, which for this time period were both numerous and intensive.
Comparison of the measured and calculated by riometric absorption time variations of proton flux during the beginning of the SCR flux increasing on October 28 is presented in Fig. 7. Calculated results for the high-latitude riometer (Abisko) is in good correspondence with direct measurements of proton flux. Diminished values of the flux calculated for two other riometers demonstrate their location at the boundary of proton penetration, where the flux is depressed significantly.
Absorption at the Polar Cap Boundary
Penetrations of solar protons into the Earth’s magnetosphere and, respectively, into the polar cap and auroral zone is regulated by the magnetic activity level. During the main phase of the magnetic storm the penetration boundaries significantly move to the equator (there were three such intervals during the studied period), and during the recovery phase—from the equator in alignment with Dst-variation [10, 11, 12].
Figure 8 presents the shift of the penetration boundary for the protons with energies of 2–4 MeV during the magnetic storms of October 2003 by records onboard the CORONAS-F satellite.
It is interesting to compare the character of proton flux decrease at the SCR penetration boundary by direct measurements and riometric data. Dependence of proton flux with energies of 14–26 MeV on magnetic latitude for two passes of the CORONAS-F satellite on October 28 during the time interval of 13:20–13:40 UT across the north polar cap and during the interval of 14:06–14:30 UT across the south polar cap is presented in Fig. 9. It can be seen that the day boundary is moved to the pole relatively the night one, which is analogous to the aurora oval shift. Besides, asymmetry of the location of the south and the north polar caps boundaries is observed: in the evening protons penetrate deeper at the north cap than at the south one, and in the morning—vice versa.
That time Scandinavian chain of riometers was at the day side of the Earth, where the value of the absorption is high, and it allowed to compare calculated values with direct measurements. In Fig. 9a x-coordinates of the letters defining the name of the stations correspond to the magnetic latitude of the stations, y-coordinates—to the calculated value of the proton flux with energies of 14–26 Mev according to expression (1). Good coincidence of the latitudinal course of SCR penetration boundary with a profile calculated on the basis of riometer chain data at the day sector.
Yakutsk chain was that time at the night side. As seen from the figure the Tiksi station is situated in the zone of free penetration of the protons. But maximum value of absorption in Tiksi did not exceed 0,5 dB which gives the calculated value of the flux an order of magnitude lower than measured by the satellite’s instruments both in the south and in the north hemispheres (letter T). Apparently, at that moment an asymmetry of the polar cap existed, but it was not observed at the pass of one of the satellites. Forty minutes later absorption in Tiksi increased up to the standard value, which is comparable with particles’ flux measured in the cap.
Analogous graph for the passes of October 30 at about 12:00 UT is presented in Fig. 9b. That time it was a break in storms, so magnetic activity was limited. There is no difference between the location of the day boundaries of the south and the north hemispheres, and shift of the night boundaries kept with the same sign, although it diminished approximately to 2∘. Riometric measurements are in a good agreement with direct measurements of protons. Relative location of Scandinavian chain of riometers and satellite’s trajectory were advantageous for comparison which is seen from Fig. 2.
It should be noted that in this chapter we did not raise a problem of studying of penetration boundaries location at different longitudinal sectors. In order to compare them correctly it is necessary to use L–B coordinates or invariant latitude. Using of geomagnetic coordinates is imposed by the task of comparison of direct measurements with riometric absorption.
PCA in daytime (Sun-lighted ionosphere) is associated with proton flux by quadratic law. Coupling coefficients for absorption dependence of proton flux measured by the instruments onboard the CORONAS-F satellite were found.
It was shown that not only protons with energies over 10–15 MeV but also protons with energies of 1–5 MeV contribute essentially to absorption. In two cases when time course in the low channel diverged from the higher channels riometric absorption followed the variations in the low-energy channel.
Numeric relation between the proton flux and PCA absorption at the night (unlighted) side of the Earth was found. It was shown that ratio of the values of the absorption at the day and the night parts of the polar cap at equal solar proton flux varies within the range of 2–4.
This work was supported in part by the grants of INTAS (Grant 03-51-5359) and RFBR (Grants 03-05-65670 and 06-05-64225).
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