Relativistic electron acceleration during HILDCAA events: are precursor CIR magnetic storms important?
© Hajra et al. 2015
Received: 21 June 2015
Accepted: 22 June 2015
Published: 7 July 2015
We present a comparative study of high-intensity long-duration continuous AE activity (HILDCAA) events, both isolated and those occurring in the “recovery phase” of geomagnetic storms induced by corotating interaction regions (CIRs). The aim of this study is to determine the difference, if any, in relativistic electron acceleration and magnetospheric energy deposition. All HILDCAA events in solar cycle 23 (from 1995 through 2008) are used in this study. Isolated HILDCAA events are characterized by enhanced fluxes of relativistic electrons compared to the pre-event flux levels. CIR magnetic storms followed by HILDCAA events show almost the same relativistic electron signatures. Cluster 1 spacecraft showed the presence of intense whistler-mode chorus waves in the outer magnetosphere during all HILDCAA intervals (when Cluster data were available). The storm-related HILDCAA events are characterized by slightly lower solar wind input energy and larger magnetospheric/ionospheric dissipation energy compared with the isolated events. A quantitative assessment shows that the mean ring current dissipation is ~34 % higher for the storm-related events relative to the isolated events, whereas Joule heating and auroral precipitation display no (statistically) distinguishable differences. On the average, the isolated events are found to be comparatively weaker and shorter than the storm-related events, although the geomagnetic characteristics of both classes of events bear no statistically significant difference. It is concluded that the CIR storms preceding the HILDCAAs have little to do with the acceleration of relativistic electrons. Our hypothesis is that ~10–100-keV electrons are sporadically injected into the magnetosphere during HILDCAA events, the anisotropic electrons continuously generate electromagnetic chorus plasma waves, and the chorus then continuously accelerates the high-energy portion of this electron spectrum to MeV energies.
KeywordsHILDCAAs High-speed streams CIRs Chorus plasma waves Radiation belt Magnetospheric relativistic electrons Solar wind Geomagnetic storms
Relativistic (MeV) electrons in the Earth’s outer radiation belt (L > 3.5) can at times have orders of magnitude variations on time scales of few minutes to several days. Earlier studies have indicated that the largest flux variations occur during geomagnetic storms (Baker et al. 1994; Li et al. 1997). Typically, relativistic electron fluxes decrease during the geomagnetic storm main phase and recover during the storm recovery phase (Baker et al. 1994; Onsager et al. 2002; Horne et al. 2009). Intense relativistic electron flux variations have also been noted during high-speed solar wind streams (HSSs) (Paulikas and Blake 1979).
The HSSs interacting with upstream slower-speed streams form high magnetic field regions called corotating interaction regions or CIRs (Smith and Wolfe 1976; Pizzo 1985; Balogh et al. 1999). CIRs interacting with the Earth’s magnetosphere can also cause magnetic storms, albeit somewhat weak storms, with typical intensities Dst >−100 nT (Tsurutani et al. 1995; Tsurutani and Gonzalez 1997; Alves et al. 2006). High-intensity long-duration continuous AE activity (HILDCAA: Tsurutani and Gonzalez 1987; Hajra et al. 2013) events created by some HSSs following the CIRs have been shown to lead to the acceleration of relativistic electrons within the Earth’s outer radiation belt (Tsurutani et al. 2006; Kasahara et al. 2009; Hajra et al. 2014a, 2015).
It is the purpose of this study to determine the possible effects of geomagnetic storms preceding HSS HILDCAA events on the acceleration of relativistic electrons. To do this, we will compare relativistic electron acceleration occurring during isolated HILDCAA events which occur in the absence of any significant storm signatures to those occurring after geomagnetic storm main phases. The geomagnetic characteristics and magnetospheric energy budget of the two classes of the events will also be compared.
HILDCAA events are distinguished by four strictly defined criteria (Tsurutani and Gonzalez 1987). These are intense auroral activity intervals characterized by peak AE intensity greater than 1000 nT, and a minimum of 2 days duration where AE does not drop below 200 nT for more than 2 h at a time. HILDCAAs, by definition, occur outside the main phases of geomagnetic storms. We use the above definitions of a HILDCAA in the present study. A geomagnetic storm is detected as a decrease in Dst with peak Dst ≤−50 nT (Akasofu 1981; Gonzalez et al. 1994), a definition used in this study.
Hajra et al. (2013) identified 133 HILDCAA events occurring during a ~3.5 solar cycle interval, from 1975 through 2011 using high-resolution AE (1 min) and Dst (1 h) data. The 43 HILDCAA events occurring during 1995 to 2008 (solar cycle SC 23) are used for the present study. The HILDCAA events were separated into storm-preceded HILDCAAs (SH) and non-storm or isolated HILDCAAs (H). The SH-events are those HILDCAAs which started after the end of geomagnetic storm main phase and occurred well inside the storm recovery phase. The geomagnetic storms preceding these SH-events were driven by CIRs as discussed previously. On the other hand, HILDCAAs not preceded by any storm main phase were also identified and are called H-events. Among the 43 events in this study, 11 were SH-events and 32 were H-events.
To study the magnetospheric energy budget, we estimated the kinetic energy of the solar wind, the magnetospheric energy input, as well as the dissipation of the energy in the inner magnetosphere-ionosphere system. The solar wind ram kinetic energy per unit time (U sw) is given by: N sw V sw 3 R CF 2. In this expression, N sw and V sw are the mass density and speed of the solar wind plasma, respectively. R CF is the distance from the Earth’s center to the subsolar location of dayside magnetopause, known as Chapman-Ferraro magnetopause distance (Chapman and Ferraro 1931; Ferraro 1952; Spreiter et al. 1966; Holzer and Slavin 1979; Sibeck et al. 1991; Monreal-MacMahon and Gonzalez 1997; Shue et al. 1997; Shue and Chao 2013). The energy transfer rate from the solar wind to the magnetosphere is estimated by the modified Akasofu epsilon parameter (ε*): V swBo2sin4(θ/2)R CF 2 (Perreault and Akasofu 1978), where Bo is the interplanetary magnetic field (IMF) magnitude and θ is the clock angle of the IMF orientation perpendicular to the Sun–Earth line (X-axis) in geocentric solar magnetospheric (GSM) coordinates. The magnetospheric input energy is dissipated within the magnetosphere/ionosphere system mainly by Joule heating, auroral precipitation, and ring current injection. The Joule heating rate (U J) is calculated using the Knipp et al. (2004) expression: a|PC| + bPC2 + c|SYM-H| + dSYM-H2, where PC is the polar cap potential index and SYM-H is the horizontal component of the symmetric ring current. The constants (a, b, c, and d) depend on the (northern hemispheric) seasons. To obtain a U J global estimate for both hemispheres, double northern hemispheric values are used for the equinoxes, while the summer and winter estimates are added for both summer and winter months. The auroral precipitation rate (U A) is obtained from NOAA/TIROS satellite measurements of high-latitude precipitating electron and ion fluxes with energies from 50 eV (or 300 eV) to 20 keV (Foster et al. 1986; Fuller-Rowell and Evans 1987; Emery et al. 2006). The global U A value is calculated by adding a southern hemisphere estimate to a northern hemisphere estimate. We assume the ring current energy injection rate (U R) to be dSYM-H*/dt + SYM-H*/τ (Akasofu 1981), where SYM-H* is the solar wind pressure corrected SYM-H index (Burton et al. 1975) with induced ground current and magnetotail current effects removed (Turner et al. 2001). In the above expression, τ is the average ring current decay time. This is taken as ~8 h for the present study (Yokoyama and Kamide 1997; Guo et al. 2011). The total input and dissipation energies, e.g., solar wind kinetic energy (E sw), magnetospheric input energy (Eε*), Joule dissipation energy (E J), auroral precipitation energy (E A), and ring current energy (E R), are calculated by time-integrating the power terms: U sw, ε*, U J, U A, and U R, respectively. It may be mentioned that the above-described methodology of estimation of magnetospheric/ionospheric energy budget has been previously used during geomagnetic storms, substorms, and HILDCAAs with good results (e.g., Turner et al. 2006, 2009; Guo et al. 2011, 2012; Hajra et al. 2014b).
The integrated fluxes of electrons with energy E > 0.6, E > 2.0, and E > 4.0 MeV at geosynchronous orbit (L = 6.6) are analyzed to study the radiation belt effects of HILDCAAs. The data (1-min time resolution) were measured by the Geostationary Operational Environment Satellite (GOES) instrumentation (Onsager et al. 1996). For more information, we refer the reader to http://www.ngdc.noaa.gov/stp/satellite/goes/dataaccess.html. Data for events during 1995–2002 are obtained from GOES-8 and those for 2003–2008 events from GOES-12. For the statistical analysis on the relativistic electron fluxes, running daily averages of the 1 min data are used in order to remove diurnal variations (e.g., Turner and Li 2008).
The Cluster 1 spacecraft is used to study possible occurrence of whistler-mode waves during its perigee passages through the magnetosphere (Santolik et al. 2014a) embedded inside the time intervals of HILDCAA events. We use continuous measurements of the Spatio-Temporal Analysis of Field Fluctuations–Spectrum Analyzer (STAFF-SA) instrument (Cornilleau-Wehrlin et al. 1997). The presence of whistler-mode emissions is identified from the measured power-spectral densities and from analysis of the wave polarization (Santolik et al. 2001, 2002; Santolik and Gurnett 2002). High-resolution waveform observations of the Wideband Data (WBD) instrument (Gurnett et al. 2001) have been used to verify presence of the discrete frequency–time structures of chorus wave packets (Santolik et al. 2014b).
The AE (1 min), Dst (1 h) and SYM-H (1-min resolution symmetric horizontal component of ring current/Dst) indices are obtained from the World Data Center for Geomagnetism, Kyoto, Japan (http://wdc.kugi.kyoto-u.ac.jp). The solar wind data (1-min resolution) are obtained from the OMNI website (http://omniweb.gsfc.nasa.gov). The latter data have already been time-shifted to coincide with solar wind convective times for impingement at the nose of the magnetopause.
Event case studies
The onset of the HILDCAA event coincides with a north-to-southward turning of the IMF Bz component, which is often a feature of HILDCAAs (Hajra et al. 2013, 2014c). The entire HILDCAA interval is characterized by high-frequency Bz variation between north and southward directions, which have been shown to be interplanetary Alfvén waves in many previous works (Tsurutani et al. 1985, 1990, 2011a, 2011b; Tsurutani and Gonzalez 1987; Echer et al. 2011).
The three panels below the topmost panel of Fig. 1 show the variations of the integrated electron fluxes (in unit of cm−2 s−1 sr−1, hereafter called flux unit, FU) at three energy levels: E > 0.6 MeV (E1), E > 2.0 MeV (E2), and E > 4.0 MeV (E3), respectively. The HILDCAA initiation time (the vertical dash-dot line) corresponds to an electron flux “dropout”. This dropout is most prominent for E > 0.6 MeV electrons. The E > 0.6 MeV electrons begin to increase at ~0343 UT on day 158 which is ~21.7 h after initiation of the HILDCAA (at ~0602 UT on day 157). The E > 2.0 MeV electrons start to increase ~1 day and 2 h after HILDCAA initiation. The E > 4.0 MeV electron fluxes begin to increase ~2 days and 22 h after HILDCAA initiation. During the HILDCAA interval, the E > 0.6, E > 2.0, and E > 4.0 MeV electron fluxes are enhanced by ~15, ~100, and ~2 times of the corresponding fluxes before the HILDCAA initiation.
CIR storm-preceded HILDCAAs and isolated HILDCAAs
As mentioned in the “Methods” section, we studied 43 HILDCAA events during SC 23, from 1995 through 2008. Among them, 11 events were preceded by CIR-induced geomagnetic storms (SH-events) and 32 were non-storm/isolated events (H-events). All the HILDCAA events were associated with enhanced fluxes at three energy levels, E > 0.6, E > 2.0, and E > 4.0 MeV, compared to pre-event fluxes. Whistler-mode chorus waves were detected for Cluster 1 perigee passes through the inner magnetosphere during HILDCAAs. In this section, we will show a comparative study on the SH- and H-events.
The variations of superposed geomagnetic indices (SYM-H and AE) display significantly different features before the event initiation (zero epoch time) and little differences during the event interval between the SH- and the H-events. For the SH-events, the geomagnetic activity as displayed in the variations of SYM-H (~−48 nT) and AE (~500 nT) is quite enhanced before zero epoch time due to the previous geomagnetic storm main phase. The low peak value of the superposed variation is due to the lack of coincidence of the peak storm phases of different storms. However, a signature of the end of storm main phase ~6 h prior to the HILDCAA initiation may be noted in the temporal display of the superposed SYM-H indices. For the H-events, variations of SYM-H and AE indicate weak enhancements of ring current and auroral activity only after zero epoch time (during the HILDCAA interval). The latter interval occurs after geomagnetic calm (Tsurutani et al. 1995). During the HILDCAA interval, the SYM-H and AE values are slightly enhanced for the SH-events (~−43 and 570 nT, respectively) over those for the H-events (~−27 and 530 nT, respectively).
Average (median) characteristics of SH- and H-events
IAE (104 nT h)
3.8 ± 2.6 (3.2)
3.0 ± 1.4 (2.6)
439.6 ± 75.4 (432.2)
415.9 ± 74.8 (397.7)
3.5 ± 1.8 (2.9)
2.9 ± 0.9 (2.6)
The second panel of Fig. 5 shows the variation of IMF Bz for the SH- and H-events. In the variation of Bz, a small but significant southward component may be noted before the zero epoch time for the SH-events. This is responsible for the storm main phase. In the case of the H-events, Bz varied around 0 nT, consistent with the geomagnetic calm before event initiation, as mentioned earlier. Interestingly, the initiation of the H-events was preceded by (~3 h) prominent northward-to-southward turning of Bz. The southward Bz component (after zero epoch time) was comparatively stronger for the SH-events (peak Bz ~−2.5 nT) than for the H-events (peak Bz ~−1.9 nT). This may explain the comparatively enhanced ring current and auroral activity noted for the SH-events.
The top panel of Fig. 5 shows the variation of the E > 2.0 MeV electron fluxes. There is no significant difference in electron fluxes between the SH- and H-events. The E > 2.0 MeV electron flux curves for the SH- and H-events are quite similar to each other. In both cases, we note flux enhancements with time lags of ~1 day after the HILDCAA initiation time. The E > 0.6 and E > 4.0 MeV electron fluxes for the SH- and H-events are also similar to each other. The latter fluxes have not been shown in order to save space.
Magnetospheric and ionospheric energy deposition
Average (median) energy budget of SH- and H-events
E sw (1018 J)
Eε* (1016 J)
E J (1016 J)
E A (1016 J)
E R (1016 J)
It has been previously shown that HILDCAA events that occur during HSSs are statistically related to relativistic electron acceleration (Hajra et al. 2013, 2014a, 2015). In this paper, we have studied HILDCAA intervals that are preceded by CIR magnetic storms and isolated (not preceded by storms) HILDCAA events for the first time. We have found virtually no differences in the relativistic electron flux characteristics between the two types of events. In fact, the only difference found is in the energetics of the ring current. There is more ring current energy associated with HILDCAAs that are preceded by CIR magnetic storms. It seems reasonable to assume that this is residual energy left over from the magnetic storms.
Hajra et al. (2015) have recently shown that electromagnetic chorus waves were present during HILDCAA intervals of SC 23 where there was Cluster plasma wave data available. In the present paper, we showed three examples of possible chorus waves detected during two HILDCAA events when Cluster 1 spacecraft passed through the inner magnetosphere. Individual HILDCAA events of this type have been previously shown by Tsurutani et al. (2006) and by Kasahara et al. (2009).
We follow the hypothesis of Tsurutani et al. (2010) for the acceleration of relativistic electrons within the magnetosphere. The southward component of the interplanetary Alfvén waves embedded within HSSs (Belcher and Davis 1971) leads to magnetic reconnection at the dayside magnetopause (Tsurutani et al. 1990, 1995). The ~10–100-keV anisotropic pitch angle electrons are injected into the midnight sector of the magnetosphere (DeForest and McIlwain 1971) and generate chorus (Tsurutani and Smith 1977; Inan et al. 1978; Meredith et al. 2003, 2006) by the loss cone/temperature anisotropy instability (Kennel and Petschek 1966; Tsurutani et al. 1979; Tsurutani and Lakhina 1997). The chorus then interacts with the upper energy end of the injected electrons, ~100 keV, to produce the ~MeV energies detected in the magnetosphere (Horne and Thorne 1998; Miyoshi et al. 2003; Omura et al. 2008; Reeves et al. 2013; Thorne et al. 2013; Boyd et al. 2014). The role of sporadic ~10–100-keV electron injection and continuous chorus generation is clearly critical to the process of relativistic electron acceleration. HILDCAA intervals, which are comprised of intense and continuous substorm/convection events, clearly provide such an environment. If a HILDCAA occurs in the HSS following the CIR storm, it appears to be occurring in the storm “recovery phase”. However, it has been shown that energy is sporadically pumped into the magnetosphere throughout HILDCAAs by the magnetic reconnection from the southward component of the Alfvén waves (Tsurutani et al. 1995, 2006; Soraas et al. 2003; Hajra et al. 2014a, b, 2015). Thus the magnetosphere is not “recovering” but is in a more or less steady state with fresh energy coming in and dissipation going on. The sporadic injections of energetic, anisotropic ~10–100-keV electrons will generate chorus waves more or less continuously.
Chorus generation is reported during geomagnetic storms (Horne and Thorne 1998; Summers et al. 1998, 2002; Meredith et al. 2002; Horne et al. 2003, 2005a, 2005b, 2007). However, the particle injections during storms are deeper into the magnetosphere (stronger convection electric fields) and last for only hours of duration. On the other hand, HSS HILDCAAs can last for days to weeks (Tsurutani et al. 1995; Hajra et al. 2013), and presumably, the chorus lasts that long as well.
The ~10–100-keV electron injection during HILDCAAs is somewhat shallow, involving only the outer portion of the magnetosphere (L ~ 5 to 7: Soraas et al. 2003) due to the relatively small convection electric fields (Tsurutani et al. 2006). Thus it is surmised that the electron acceleration is taking place at L values close to geosynchronous orbit. The NOAA GOES satellites might be in ideal locations to monitor events of this type.
Two individual isolated HILDCAA events were shown to be related to relativistic E > 0.6, E > 2.0, and E > 4.0 MeV flux increases (Figs. 1 and 3). It was shown that intense whistler-mode chorus waves exist in the outer magnetosphere using simultaneous Cluster 1 satellite observations. This was typical of all of the isolated HILDCAA intervals that occurred during SC 23.
A superposed epoch analysis of HILDCAAs with preceding CIR magnetic storms and isolated HILDCAAs indicated little or no differences in relativistic electron flux characteristics (Fig. 5).
The solar wind energy input and the energy dissipation (Joule heating, auroral energy, and ring current energy) for CIR magnetic storm preceding HILDCAAs and for isolated HILDCAAs are virtually the same, except that CIR storm-preceded HILDCAA events have ~34 % higher ring current dissipation than isolated HILDCAA events (Fig. 6 and Table 2).
corotating interaction region
Geostationary Operational Environment Satellite
geocentric solar magnetospheric
high-intensity long-duration continuous AE activity
high-speed solar wind stream
interplanetary magnetic field
Spatio-Temporal Analysis of Field Fluctuations–Spectrum Analyzer
The work of RH is financially supported by Fundacao de Amparo a Pesquisa do Estado de Sao Paulo (FAPESP) through post-doctoral research fellowship at INPE. EE would like to thank to the Brazilian CNPq (301233/2011-0) agency for financial support. Portions of this research were performed at the Jet Propulsion Laboratory, California Institute of Technology under contract with NASA. The Arecibo Observatory is operated by SRI International in collaboration with the Universities Space Research Association and Universidade Metropolitana under a cooperative agreement with National Science Foundation, award number 1160876. OS acknowledges funding from Premium Academiae, and grants LH12231 and LH14010.
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