Rajkumar Hajra

Relativistic electron acceleration during high‐intensity, long‐duration, continuous AE activity (HILDCAA) events: Solar cycle phase dependences

High-intensity, long-duration, continuous AE activity (HILDCAA) intervals during solar cycle 23 (1995–2008) have been studied by a superposed epoch analysis. It was found that HILDCAA intervals order the solar wind velocity, temperature and density (characteristic of high-speed solar wind intervals), the polar cap potential, and various other geomagnetic indices well. The interplanetary magnetic field Bz is generally negative, and the Newell solar wind coupling function is high during HILDCAA events. The HILDCAA intervals are well correlated with an enhancement of magnetospheric relativistic (E > 2 MeV) electron fluxes observed at geosynchronous orbit with a delay of ~1.5 days from the onset of the HILDCAAs. The response of the energetic electrons to HILDCAAs is found to vary with solar cycle phase. The initial electron fluxes are lower for events occurring during the ascending and solar maximum (AMAX) phases than for events occurring during the descending and solar minimum (DMIN) phases. The flux increases for the DMIN phase events are >50% larger than for the AMAX phase events. Although the solar wind speeds during the DMIN phases were slightly higher and lasted longer than during the AMAX phases, no other significant solar wind differences were noted. It is concluded that electrons are accelerated to relativistic energies most often and most efficiently during the DMIN phases of the solar cycle. We propose two possible solar UV mechanisms to explain this solar cycle effect.

RELATIVISTIC (E > 0.6, > 2.0, AND > 4.0 MeV) ELECTRON ACCELERATION AT GEOSYNCHRONOUS ORBIT DURING HIGH-INTENSITY, LONG-DURATION, CONTINUOUS AE ACTIVITY (HILDCAA) EVENTS

Radiation-belt relativistic (E > 0.6, > 2.0, and > 4.0?MeV) electron acceleration is studied for solar cycle 23 (1995-2008). High-intensity, long-duration, continuous AE activity (HILDCAA) events are considered as the basis of the analyses. All of the 35 HILDCAA events under study were found to be characterized by flux enhancements of magnetospheric relativistic electrons of all three energies compared to the pre-event flux levels. For the E > 2.0?MeV electron fluxes, enhancement of >50% occurred during 100% of HILDCAAs. Cluster-4 passes were examined for electromagnetic chorus waves in the 5 0.6, > 2.0, and > 4.0?MeV electrons occurred ~1.0?day, ~1.5?days, and ~2.5?days after the statistical HILDCAA onset, respectively. The statistical acceleration rates for the three energy ranges were ~1.8?? 105, 2.2?? 103, and 1.0?? 101 cm?2 s?1 sr?1 d?1, respectively. The relativistic electron-decay timescales were determined to be ~7.7, 5.5, and 4.0?days for the three energy ranges, respectively. The HILDCAAs were divided into short-duration (D ? 3?days) and long-duration (D > 3?days) events to study the dependence of relativistic electron variation on HILDCAA duration. For long-duration events, the flux enhancements during HILDCAAs with respect to pre-event fluxes were ~290%, 520%, and 82% for E > 0.6, > 2.0, and > 4.0?MeV electrons, respectively. The enhancements were ~250%, 400%, and 27% respectively, for short-duration events. The results are discussed with respect to the current understanding of radiation-belt dynamics.

Solar wind‐magnetosphere energy coupling efficiency and partitioning: HILDCAAs and preceding CIR storms during solar cycle 23

A quantitative study on the energetics of the solar wind-magnetosphere-ionosphere system during High-Intensity, Long-Duration, Continuous AE Activity (HILDCAA) events for solar cycle 23 (from 1995 through 2008) is presented. For all HILDCAAs, the average energy transferred to the magnetospheric/ionospheric system was ~6.3 ×1016 J, and the ram kinetic energy of the incident solar wind was ~7.1 ×1018 J. For individual HILDCAA events the coupling efficiency, defined as the ratio of the solar wind energy input to the solar wind kinetic energy, varied between 0.3% and 2.8%, with an average value of ~0.9%. The solar wind coupling efficiency for corotating interaction region (CIR)-driven storms prior to the HILDCAA events was found to vary from ~1% to 5%, with an average value of ~2%. Both of these values are lower than the> 5% coupling efficiency noted for interplanetary coronal mass ejection (and sheath)-driven magnetic storms. During HILDCAAs, ~67% of the solar wind energy input went into Joule heating, ~22% into auroral precipitation, and ~11% into the ring current energy. The CIR-storm Joule heating (~49%) was noticeably less than that during HILDCAAs, while the ring current energies were comparable for the two. Joule dissipation was higher for HILDCAAs that followed CIR-storms (88%) than for isolated HILDCAAs (~60%). Possible physical interpretations for the statistical results obtained in this paper are discussed.

Heliospheric plasma sheet (HPS) impingement onto the magnetosphere as a cause of relativistic electron dropouts (REDs) via coherent EMIC wave scattering with possible consequences for climate change mechanisms

A new scenario is presented for the cause of magnetospheric relativistic electron decreases (REDs) and potential effects in the atmosphere and on climate. High density solar wind heliospheric plasmasheet (HPS) events impinge onto the magnetosphere, compressing it along with remnant noon-sector outer-zone magnetospheric ~10-100 keV protons. The betatron accelerated protons generate coherent EMIC waves through a temperature anisotropy (T┴/T|| > 1) instability. The waves in turn interact with relativistic electrons and cause the rapid loss of these particles to a small region of the atmosphere. A peak total energy deposition of ~3 x 1020 ergs is derived for the precipitating electrons. Maximum energy deposition and creation of electron-ion pairs at 30-50 km and at < 30 km altitude are quantified. We focus the readers’ attention on the relevance of this present work to two climate change mechanisms. Wilcox et al. [1973] noted a correlation between solar wind heliospheric current sheet (HCS) crossings and high atmospheric vorticity centers at 300 mb altitude. Tinsley et al. [1994] has constructed a global circuit model which depends on particle precipitation into the atmosphere. Other possible scenarios potentially affecting weather/climate change are also discussed.

Relativistic electron acceleration during HILDCAA events: are precursor CIR magnetic storms important?

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.

Supersubstorms (SML < −2500 nT): Magnetic storm and solar cycle dependences

We study extremely intense substorms with SuperMAG AL (SML) peak intensities < −2500 nT (“supersubstorms”/SSSs) for the period from 1981 to 2012. The SSS events were often found to be isolated SML peaks and not statistical fluctuations of the indices. The SSSs occur during all phases of the solar cycle with the highest occurrence (3.8 year−1) in the descending phase. The SSSs exhibited an annual variation with equinoctial maximum altering between spring in solar cycle 22 and fall in solar cycle 23. The occurrence rate and strength of the SSSs did not show any strong relationship with the intensity of the associated geomagnetic storms. All SSS events were associated with strong southward interplanetary magnetic field Bs component. The Bs fields were part of interplanetary magnetic clouds in 46% and of interplanetary sheath fields in 54% of the cases. About 77% of the SSSs were associated with small regions of very high density solar wind plasma parcels or pressure pulses impinging upon the magnetosphere. Comments on how SSS events may cause power outages at Earth are discussed at the end of the paper.

High‐speed solar wind stream effects on the topside ionosphere over Arecibo: A case study during solar minimum

The impact of a high-speed solar wind stream (HSS) on the topside near-equatorial ionosphere (Arecibo: 28.17°N, L = 1.3) is investigated for the first time. Although the HSS did not lead to any significant geomagnetic storm activity, the ionosphere over Arecibo became hotter and expanded significantly in altitude as compared to a non-HSS interval. The O+/H+ transition height hT increased by ~200 km in the daytime and by ~100 km at night. At the hT, the peak ionospheric electron and ion temperatures increased by ~200-500 K during day and by ~50-70 K at night. While the O+ ion concentration exhibited an overall enhancement, deep penetration of the H+ ions below hT are observed during the day. The noontime peak electron density was ~4 times higher during the HSS event compared to the non-HSS interval. We present three possible mechanisms to explain this topside ionospheric heating.

Plasma source and loss at comet 67P during the Rosetta mission

Context. The Rosetta spacecraft provided us with a unique opportunity to study comet 67P/Churyumov-Gerasimenko from a close perspective and over a two-year time period. Comet 67P is a weakly active comet. It was therefore unexpected to find an active and dynamic ionosphere where the cometary ions were largely dominant over the solar wind ions, even at large heliocentric distances. Aims. Our goal is to understand the different drivers of the cometary ionosphere and assess their variability over time and over the different conditions encountered by the comet during the Rosetta mission. Methods. We used a multi-instrument data-based ionospheric model to compute the total ion number density at the position of Rosetta. In-situ measurements from the Rosetta Orbiter Spectrometer for Ion and Neutral Analysis (ROSINA) and the Rosetta Plasma Consortium (RPC)–Ion and Electron Sensor (IES), together with the RPC–LAngmuir Probe instrument (LAP) were used to compute the local ion total number density. The results are compared to the electron densities measured by RPC–Mutual Impedance Probe (MIP) and RPC–LAP. Results. We were able to disentangle the physical processes responsible for the formation of the cometary ions throughout the two-year escort phase and we evaluated their respective magnitudes. The main processes are photo-ionization and electron-impact ionization. The latter is a significant source of ionization at large heliocentric distance (> 2 au) and was predominant during the last four months of the mission. The ionosphere was occasionally subject to singular solar events, temporarily increasing the ambient energetic electron population. Solar photons were the main ionizer near perihelion at 1.3 au from the Sun, during summer 2015.

Impact of a cometary outburst on its ionosphere Rosetta Plasma Consortium observations of the outburst exhibited by comet 67P/Churyumov-Gerasimenko on 19 February 2016

We present a detailed study of the cometary ionospheric response to a cometary brightness outburst using in situ measurements for the first time. The comet 67P/Churyumov-Gerasimenko (67P) at a heliocentric distance of 2.4 AU from the Sun, exhibited an outburst at ∼1000 UT on 19 February 2016, characterized by an increase in the coma surface brightness of two orders of magnitude. The Rosetta spacecraft monitored the plasma environment of 67P from a distance of 30 km, orbiting with a relative speed of ∼0.2 m s −1. The onset of the outburst was preceded by pre-outburst decreases in neutral gas density at Rosetta, in local plasma density, and in negative spacecraft potential at ∼0950 UT. In response to the outburst, the neutral density increased by a factor of ∼1.8 and the local plasma density increased by a factor of ∼3, driving the spacecraft potential more negative. The energetic electrons (tens of eV) exhibited decreases in the flux of factors of ∼2 to 9, depending on the energy of the electrons. The local magnetic field exhibited a slight increase in amplitude (∼5 nT) and an abrupt rotation (∼36.4 •) in response to the outburst. A weakening of 10–100 mHz magnetic field fluctuations was also noted during the outburst, suggesting alteration of the origin of the wave activity by the outburst. The plasma and magnetic field effects lasted for about 4 h, from ∼1000 UT to 1400 UT. The plasma densities are compared with an ionospheric model. This shows that while photoionization is the main source of electrons, electron-impact ionization and a reduction in the ion outflow velocity need to be accounted for in order to explain the plasma density enhancement near the outburst peak.