Space Physics Seminar - winter-2022
ARTEMIS at 10 Years: Past Science Highlights and Future Goals
Jan. 7, 2022
3:30 p.m. - 5 p.m.
Zoom
Presented By:
- Andrew Poppe - UC Berkeley Space Sciences Lab
Subscribe to Calendar
NASA’s THEMIS-ARTEMIS mission entered lunar orbit in mid-2011 and since then, has returned a treasure trove of data for studies of the lunar plasma environment. To date, the ARTEMIS mission has resulted in over 200 total publications, nearly 100 of which are directly related to the lunar plasma environment and/or the subsequent effects of the Moon on its surroundings. Now past its 10-year mark in lunar orbit, I look back and highlight results from the ARTEMIS mission, focusing on work related to the Moon’s interaction with ambient plasma. In particular, I will discuss ARTEMIS studies of: (1) the lunar wake as an example for fundamental space-plasma processes throughout the solar system; (2) the lunar neutral and ionized exosphere, which both responds to and perturbs the near-lunar space environment; and, (3) electrostatic lunar surface charging, driven by a combination of solar wind and magnetospheric plasmas, as well as interplanetary dust. ARTEMIS has made broad and fundamental contributions to our understanding of each of these areas, transforming our view of the Moon from a simple passive absorber to a host of rich and complex plasma processes that actively perturb the lunar environment. Finally, I will conclude by discussing on-going and future avenues of research with ARTEMIS, including upcoming synergies with the NASA Heliophysics Division’s HERMES payload to be hosted on the Lunar Gateway.
Magnetic Switchbacks in the Young Solar Wind: Parker Solar Probe observations
Jan. 14, 2022
3:30 p.m. - 5 p.m.
Zoom
Presented By:
- Olekiy Agapitov - UC Berkeley Space Sciences Lab
Subscribe to Calendar
A major discovery of Parker Solar Probe (PSP) was the presence of large numbers of localized increases in the radial solar wind speed and associated sharp deflections of the magnetic field - switchbacks (SB). A possible generation mechanism of SBs is through magnetic reconnection between open and closed magnetic flux near the solar surface termed interchange reconnection that leads to the ejection of flux ropes (FR) into the solar wind. The role of FRs merging in controlling the structure of SB in the solar wind is explored through direct observations, analytic analysis, and numerical simulations. Observations also suggest that SBs undergo merging, which is shown to be energetically favorable to reduce the strength of the wrapping magnetic field and that this drives the observed elongation of SBs. A further consequence is a resulting dominance of the axial magnetic field within SBs that leads to the characteristic sharp rotation of the magnetic field into the axial direction at the SB boundary that is revealed in observations. Observations by PSP reveal the existence of intensive plasma wave bursts with frequencies below 0.1 fce (from tens of Hz to 150 Hz in the spacecraft frame) collocated with the switchbacks boundaries. Sunward propagation with depletion of magnetic field magnitude leads to a significant Doppler frequency downshift of whistler waves from 200-300 Hz (0.2-0.5 fce) down to 20-80 Hz in the spacecraft frame. Their peak amplitudes can be as large as 2 to 4 nT. Such values represent approximately 10-20% of the background magnetic field. We have evaluated the properties of these waves collocated with dips of magnetic field related to switchback boundaries, the mechanisms of wave generation: the generation of these waves is supported by the modified electron distribution with increased transverse temperature anisotropy inside the magnetic holes; and the effects on solar wind suprathermal particles from interaction with these waves: sunward propagating whistler waves efficiently interact with the high energy solar wind electrons (in the energy range up to 1 keV) scattering the strahl population of suprathermal electrons into a halo population due to the most efficient cyclotron resonance interaction.
Magnetic reconnection at the Heliospheric Current sheet very close to the Sun
Jan. 21, 2022
3:30 p.m. - 5 p.m.
Zoom
Presented By:
- Tai Phan - UC Berkeley Space Sciences Lab
Subscribe to Calendar
During its first eight orbits around the Sun, Parker Solar Probe (PSP) crossed the large-scale Heliospheric Current Sheet (HCS) multiple times and provided unprecedented detailed plasma and field observations of the near-Sun HCS. We report the common detections by PSP of reconnection exhaust signatures in the HCS at heliocentric distances of 16-107 solar radii. Both sunward and antisunward-directed reconnection exhausts were observed. In the sunward reconnection exhausts, PSP detected counterstreaming strahl electrons, indicating that HCS reconnection resulted in the formation of closed magnetic field lines with both ends connected to the Sun. In the antisunward exhausts, PSP observed dropouts of strahl electrons, consistent with the reconnected HCS field lines being disconnected from the Sun. Ion and electron signatures of the reconnection separatrix layers are also observed adjacent to some exhausts. The common detection of reconnection in the HCS suggests that reconnection is almost always active in the HCS near the Sun. Furthermore, the occurrence of multiple long-duration partial crossings of the HCS suggests that HCS reconnection could produce chains of large bulges with spatial dimensions of up to several solar radii. The finding of the prevalence of reconnection in the HCS is somewhat surprising since PSP has revealed that the HCS is much thicker than the kinetic scales required for reconnection onset. Thus, the PSP findings suggest that large-scale dynamics either locally in the solar wind or within the coronal source of the HCS (e.g., at the tip of helmet streamers) plays a critical role in triggering reconnection onset
Energetic Electron Precipitation into Earth’s Atmosphere Driven by Electromagnetic Ion Cyclotron Waves
Jan. 28, 2022
3:30 p.m. - 5 p.m.
Zoom
Presented By:
- Louisa Capannolo - Boston University
Subscribe to Calendar
The Earth’s outer electron radiation belt is a highly dynamic environment, governed by the balance between processes of acceleration, transport and loss. One important mechanism that can significantly deplete the outer radiation belt is electron precipitation. This phenomenon occurs when electrons are no longer trapped by the magnetic field and fall along the magnetic field lines into the Earth’s atmosphere. Here, the energy input due to electron precipitation can affect the chemistry of the atmosphere, changing the conductivity as well as potentially causing ozone reduction. Electromagnetic Ion Cyclotron (EMIC) waves are known to precipitate energetic electrons via pitch-angle scattering. These waves (frequencies of ~0.1–5 Hz) are excited during injections from the magnetotail or solar wind pressure changes, and are often associated with spatially localized dropouts of the outer radiation belt flux at ~MeV energies. EMIC waves are indeed efficient in driving electron precipitation in the dusk sector and can also produce isolated proton aurora. However, several key properties of EMIC-driven electron precipitation (e.g., energy range, spatial extent, location, etc.) are not fully understood yet. In this talk, I will show examples of electron precipitation associated with EMIC waves using data from multiple low-Earth-orbiting satellites and CubeSats. Taking advantage of magnetic conjunctions between equatorial and low-altitude satellites as well as theory simulations, we demonstrate that EMIC waves can not only precipitate ~MeV electrons, but also sub-MeV and even sub-relativistic electrons as well –a wider energy range than previously thought. The collection of the EMIC-driven electron precipitation events also shows that these waves typically cause radially localized precipitation from the pre-dusk sector to post-midnight.
Active Control of the Radiation Belt Particle Populations with Ionospheric Amplification of VLF Waves
Feb. 4, 2022
3:30 p.m. - 5 p.m.
Zoom
Presented By:
- Paul Bernhardt - University of Alaska, Fairbanks
Subscribe to Calendar
Ground-based VLF transmitters located around the world generate signals that leak through the bottom side of the ionosphere in the form of whistler mode waves. Wave and particle measurements on satellites have observed that these man-made VLF waves can be strong enough to scatter trapped energetic electrons into low pitch angle orbits, causing loss by absorption in the lower atmosphere. This precipitation loss process is greatly enhanced by intentional amplification of the whistler waves in the ionosphere using a newly discovered process called Rocket Exhaust Driven Amplification (REDA). Satellite measurements of REDA have shown between 30 and 50 dB intensification of VLF waves in space using a 60-second burn of the 150 g/s thruster on the Cygnus satellite that services the International Space Station (ISS) [Bernhardt et al. 2021; Bernhardt 2021]. This controlled amplification process is adequate to deplete the energetic particle population in the radiation belts in a few minutes rather than the multi-day period it would take naturally. Numerical simulations of the pitch angle diffusion for radiation belt particles use the UCLA quasi-linear Fokker Planck model (QLFP) to assess the impact of REDA on radiation belt remediation (RBR) of newly injected energetic electrons [Bernhardt et al., 2022]. The simulated precipitation fluxes of energetic electrons are applied to models of D-region electron density, bremsstrahlung x-rays and optical emissions for predictions of the modified environment that can be observed with satellite and ground-based sensors.
Uncertainty in solar wind forcing and its relation to polar cap potential saturation
Feb. 11, 2022
3:30 p.m. - 5 p.m.
Zoom
Presented By:
- Nithin Sivadas - NASA Goddard Space Flight Center
Subscribe to Calendar
Extreme space weather events occur during intervals of strong solar wind electric fields. Curiously during these intervals, their impact on measures of the Earth's response, like the polar cap index, is not as high as expected. Theorists have put forward a host of explanations for this saturation effect, but there is no consensus. Here we show that the saturation is merely a perception created by uncertainty in the solar wind measurements, especially in the measurement times. Correcting for the uncertainty reveals that extreme space weather events elicit a ~300% larger impact than previously thought. Furthermore, they point to a surprisingly general result relevant to any correlation study: uncertainty in the measurement time can cause a system's linear response to be perceived as non-linear.