High‐Intensity, Long‐Duration, Continuous AE Activity (HILDCAA) events are studied using long‐term geomagnetic and solar wind/interplanetary databases. We use the strict definition of a HILDCAA ...event, that it occurs outside of the main phase of a magnetic storm, the peak AE is >1000 nT, and the duration is at least 2 days long. One hundred thirty‐three events have been identified from the AE indices in the 1975 to 2011 interval, a ~3½ solar cycle span. Of the 133 events, 99 had simultaneous interplanetary data available. The overwhelmed majority (94%) of these latter cases were associated with high‐speed solar wind stream (HSS) events. The remaining 6% of the cases occurred after the passage of interplanetary coronal mass ejections (ICMEs). The HSS‐related events were typically associated with large interplanetary magnetic field (IMF) Bz variances. The ICME‐related events were characterized by steady southward Bz intervals or low‐frequency fluctuations, both of which we view as possible different interplanetary phenomena. HILDCAA events have been found to have their largest occurrence frequency in the solar cycle descending phase (~6.8/year) with the second largest at solar minimum (~3.5/year). The occurrence frequencies were considerably lower in the ascending phase (~2.5/year) and at solar maximum (~2.2/year). Thus, HILDCAAs can occur during all phases of the solar cycle, with the descending phase approximately three times more likely to have an event than at solar maximum and the ascending phase. The HILDCAA events that occurred in the declining phase and at solar minimum were >20% longer in duration than those in the ascending phase and solar maximum, respectively. The events during the recent solar and geomagnetic minima, 2007–2009, were, on the average, ~17% and 14% weaker in peak AE than the events during the previous two minima of 1995–1997 and 1985–1987, respectively. The recent minimum events were ~35% and 41% shorter in durations, respectively, than the events during those previous minima. The yearly occurrence of the events exhibited statistically significant correlation (>0.70) with yearly average speed and number of HSSs. No seasonal dependence of HILDCAA was noted.
Key Points
94% of HILDCAAs were associated with HSSs/CIRs and 6% with ICMEs
The HSSs were associated with large variances in IMF Bz
Maximum HILDCAA occurrence during the solar declining phase
Abstract
The ESA’s comet chaser Rosetta has monitored the evolution of the ionized atmosphere of comet 67P/Churyumov–Gerasimenko (67P/CG) and its interaction with the solar wind, during more than ...2 yr. Around perihelion, while the cometary outgassing rate was highest, Rosetta crossed hundreds of unmagnetized regions, but did not seem to have crossed a large-scale diamagnetic cavity as anticipated. Using in situ Rosetta observations, we characterize the structure of the unmagnetized plasma found around comet 67P/CG. Plasma density measurements from RPC-MIP are analysed in the unmagnetized regions identified with RPC-MAG. The plasma observations are discussed in the context of the cometary escaping neutral atmosphere, observed by ROSINA/COPS. The plasma density in the different diamagnetic regions crossed by Rosetta ranges from ∼100 to ∼1500 cm−3. They exhibit a remarkably systematic behaviour that essentially depends on the comet activity and the cometary ionosphere expansion. An effective total ionization frequency is obtained from in situ observations during the high outgassing activity phase of comet 67P/CG. Although several diamagnetic regions have been crossed over a large range of distances to the comet nucleus (from 50 to 400 km) and to the Sun (1.25–2.4 au), in situ observations give strong evidence for a single diamagnetic region, located close to the electron exobase. Moreover, the observations are consistent with an unstable contact surface that can locally extend up to about 10 times the electron exobase.
We examine particularly intense substorms (SML less than or equal to -2500 nT), hereafter called "supersubstorms" or SSS events, to identify their nature and their magnetic storm dependences. It is ...found that these intense substorms are typically isolated events and are only loosely related to magnetic storms. SSS events can occur during super (Dst less than or equal to -250 nT) and intense (-100 nT greater than or equal to Dst >-250) magnetic storms. SSS events can also occur during nonstorm (Dst greater than or equal to -50 nT) intervals. SSSs are important because the strongest ionospheric currents will flow during these events, potentially causing power outages on Earth. Several SSS examples are shown. SSS events appear to be externally triggered by small regions of very high density ( similar to 30 to 50 cm-3) solar wind plasma parcels (PPs) impinging upon the magnetosphere. Precursor southward interplanetary magnetic fields are detected prior to the PPs hitting the magnetosphere. Our hypothesis is that these southward fields input energy into the magnetosphere/magnetotail and the PPs trigger the release of the stored energy.
Electromagnetic ion (proton) cyclotron (EMIC) waves and whistler mode chorus are simultaneously detected in the Earth's dayside subsolar outer magnetosphere. The observations were made near the ...magnetic equator 3.1°–1.5° magnetic latitude at 1300 magnetic local time from L = 9.9 to 7.0. It is hypothesized that the solar wind external pressure caused preexisting energetic 10–100 keV protons and electrons to be energized in the T⊥ component by betatron acceleration and the resultant temperature anisotropy (T⊥>T∥) formed led to the simultaneous generation of both EMIC (ion) and chorus (electron) waves. The EMIC waves had maximum wave amplitudes of ∼6 nT in a ∼60 nT ambient field B0. The observed EMIC wave amplitudes were about ∼10 times higher than the usually observed chorus amplitudes (∼0.1–0.5 nT). The EMIC waves are found to be coherent to quasi‐coherent in nature. Calculations of relativistic ∼1–2 MeV electron pitch angle transport are made using the measured wave amplitudes and wave packet lengths. Wave coherency was assumed. Calculations show that in a ∼25–50 ms interaction with an EMIC wave packet, relativistic electron can be transported ∼27° in pitch. Assuming dipole magnetic field lines for a L = 9 case, the cyclotron resonant interaction is terminated ∼±20° away from the magnetic equator due to lack of resonance at higher latitudes. It is concluded that relativistic electron anomalous cyclotron resonant interactions with coherent EMIC waves near the equatorial plane is an excellent loss mechanism for these particles. It is also shown that E > 1 MeV electrons cyclotron resonating with coherent chorus is an unlikely mechanism for relativistic microbursts. Temporal structures of ∼30 keV precipitating protons will be ∼2–3 s which will be measurable at the top of the ionosphere.
Key Points
Simultaneous EMIC and chorus wave occurrence in the Earth's outer magnetosphere
High pitch angle transport rate of relativistic electrons by coherent EMIC waves
EMIC waves more efficient than chorus for relativistic electron pitch angle loss
Using in situ measurements from different instruments on board the Rosetta spacecraft, we investigate the properties of the newly discovered low-frequency oscillations, known as singing comet waves, ...that sometimes dominate the close plasma environment of comet 67P/Churyumov-Gerasimenko. These waves are thought to be generated by a modified ion-Weibel instability that grows due to a beam of water ions created by water molecules that outgass from the comet. We take advantage of a cometary outburst event that occurred on 2016 February 19 to probe this generation mechanism. We analyze the 3D magnetic field waveforms to infer the properties of the magnetic oscillations of the cometary ion waves. They are observed in the typical frequency range (~50 mHz) before the cometary outburst, but at ~20 mHz during the outburst. They are also observed to be elliptically right-hand polarized and to propagate rather closely (~0−50°) to the background magnetic field. We also construct a density dataset with a high enough time resolution that allows us to study the plasma contribution to the ion cometary waves. The correlation between plasma and magnetic field variations associated with the waves indicates that they are mostly in phase before and during the outburst, which means that they are compressional waves. We therefore show that the measurements from multiple instruments are consistent with the modified ion-Weibel instability as the source of the singing comet wave activity. We also argue that the observed frequency of the singing comet waves could be a way to indirectly probe the strength of neutral plasma coupling in the 67P environment.
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 electromagnetic ion cyclotron (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 × 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. () 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.
Key Points
Heliospheric plasmasheet impingements on the magnetosphere lead to relativistic electron losses
Compressing the magnetosphere causes generation of coherent EMIC waves which confine electron losses to small region of dayside ionosphere
Energy deposition and ionization at low altitudes may cause Wilcox effect and Tinsley effect
Context. The Rosetta spacecraft provided us with a unique opportunity to study comet 67P/Churyumov–Gerasimenko (67P) from a close perspective and over a 2-yr 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 2-yr 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 4 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.
In this study thermophysical properties of alloy 617 have been measured as a function of temperature using various thermal analysis techniques and complemented by structural and microstructural ...characterization. The enthalpy increment (HT−H298) has been measured using static calorimetry in the temperature range of 300–1523 K. It is found that (HT−H298) increases monotonically with temperature up to about 1000 K. However, above this temperature two distinct inflections are detected at about 1030 K and 1205 K respectively. A critical comparison of these inflections with Thermo-Calc simulations and dynamic calorimetry measurements indicates that they are associated with dissolution of M23C6, and γ′-Ni3Al phases respectively. The enthalpy data has been numerically fitted using non-linear regression analysis to obtain the temperature dependence of heat capacity. Further, the thermal diffusivity and thermal expansivity of alloy 617 have been measured using the laser flash method and thermo-mechanical analyzer in the temperature range of 300–1523 K. Finally, with the knowledge of heat capacity, density and thermal diffusivity data, thermal conductivity has also been estimated in the temperature range of 300–1523 K. The measured properties for alloy 617 have been compared with Thermo-Calc, JMatPro simulation and other literature data, which clearly establishes the role of precipitates dissolution. The various properties measured in the present study are original and new addition to the database for alloy 617, which can serve as new input for Thermo-calc based optimization. This paper also provides the insight into the temperature and composition dependence of thermal conductivity of alloy 617.
•In this study, various thermophysical properties of alloy 617 have been reported.•Experimental and theoretical approaches are used to estimate these properties.•Variation of these properties are correlated with microstructure of alloy 617.•Alloying addition and various precipitates play significant role on these data.•Alloying addition and magnetic ordering significantly govern thermal conductivity.
The interplanetary and geomagnetic characteristics of High-Intensity Long-Duration Continuous AE Activity (HILDCAA) events are studied using wavelet analysis technique. The Morlet wavelet transform ...was applied to the 1min interplanetary magnetic field (IMF) Bz component and the geomagnetic AE index during HILDCAA events. We have analyzed the AE data for the events occurring between 1975 and 2011, and the IMF Bz data (both in GSE and GSM) for the events between 1995 and 2011. We analyzed the scalograms and the global wavelet spectrum of the parameters. For 50% of all HILDCAA events, the main periodicities of the AE index are generally between 4 and 12h. For the Bz component, the main periodicities were found to be less than 8h for ~56% of times in GSM system and for ~54% of times in GSE system. It is conjectured that the periodicities might be associated with the Alfvén waves which have typical periods between 1 and 10h. The results are discussed in the light of self organized criticality theory where the physical events have the capacity of releasing a considerable amount of energy in a short interval of time.
•The most periods of the AE index during HILDCAAs are localized between 4 and 12h.•The periods most commonly found in the IMF Bz component were ≤8h.•The periods mentioned before may be associated with Alfvén waves.•The AE periods during the HILDCAA exhibit intermittent characteristics.•These characteristics of AE mentioned before may be associated with the SOC.
Long-term variations in the relativistic (∼MeV) electrons in the Earth's radiation belt are explored to study seasonal features of the electrons. An L-shell dependence of the seasonal variations in ...the electrons is reported for the first time. A clear ∼6 month periodicity, representing one/two peaks per year, is identified for 1.5–6.0 MeV electron fluxes in the L shells between ∼3.0 and ∼5.0. The relativistic electron flux variation is strongest during solar cycle descending to minimum phases, with weaker/no variations during solar maximum. If two peaks per year occur, they are largely asymmetric in amplitude. The peaks essentially do not have an equinoctial
dependence. Sometimes the peaks are shifted to solstices, and sometimes only one annual peak is observed. No such seasonal features are prominent for L<3.0 and L>5.0. The results imply varying solar/interplanetary drivers of the radiation belt electrons at different L shells. This has a potential impact on the modeling of the space environment. Plausible solar drivers are discussed.
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