Category: Seminars

Thursday 3/31 Seminar: Some Like it Hot: Plasma Heating and Impulsive Energy Release in the Solar Corona

March 28th, 2016 in Seminars

Some Like it Hot: Plasma Heating and Impulsive Energy Release in the Solar Corona

Dr. Amir Caspi
Southwest Research Institute, Boulder

Abstract:
The solar corona is hot, at temperatures of ~1-2 million Kelvin (MK) globally and up to ~10 MK above the magnetically-complex features known as active regions. This is 2-3 orders of magnitude hotter than the underlying chromosphere and photosphere, and this “coronal heating problem” remains one of the fundamental unanswered questions in solar and stellar physics. During solar flares, where enormous quantities of energy are impulsively released and explosively converted to accelerated particles and superheated plasma, tempera- tures can reach yet another ten-fold higher, but the details of these processes and how (or whether) they relate to quiescent heating remain poorly understood. Characterizing these phenomena on the Sun is important to understanding impulsive energy release in magnetized plasmas throughout the universe, but solar variability also holds crucial importance for space weather studies, as the extreme ultraviolet (EUV) and soft and hard X-ray (SXR, HXR) emission from hot coronal plasma is the primary driver of dynamical processes in Earth’s ionosphere, mesosphere, and thermosphere (ITM).
Standard flare models typically consider hot plasma as originating in the chromosphere, heated by collisional energy losses from flare-accelerated particles and “evaporating” into the corona. However, evidence is mounting that a significant amount of flare thermal plasma, especially at the hottest temperatures, is heated in situ, directly within the corona, with important implications for flare energy release mechanisms and flare energetics. During quiescence, a leading candidate theory for active region heating is the nanoflare model, whereby many tiny impulsive, flare-like events occur in rapid succession. However, these events are too small to be resolved either spatially or temporally, so it is not known where in the atmosphere the heating occurs, nor hence what physical mechanisms dominate. Consequently, while both flares and nanoflares are likely to be powered by the Sun’s magnetic field, a clear connection between these two opposite regimes of energy release has not yet been well established.
We present a brief overview of solar flares and coronal heating, and discuss results from SDO, RHESSI, and other missions using EUV and X-ray observations to study plasma heating in intense solar flares. We present recent sounding rocket observations of the quiescent Sun using a new SXR spectrometer, and discuss the MinXSS CubeSat that will make routine observations with this instrument after its imminent deployment from the ISS. We also review a recent Antarctic long-duration balloon flight to space-qualify new HXR spectrometers for flare observations. Finally, we introduce new mission concepts, including the CubIXSS 6U CubeSat and a larger-scale microsatellite, that will make breakthrough measurements of flaring and quiescent X-ray emission, using both imaging and spectroscopy, for understanding solar coronal plasma heating and its influence on Earth’s ITM.

Monday 3/28 Seminar: Magnetic fields in star forming regions: from dense core to disk scales.

March 28th, 2016 in Seminars

Magnetic fields in star forming regions: from dense core to disk scales

Josep Miquel Girart
Institut de Ciencies de L’espai

Monday, March 28, 2016
3:15 pm – Refreshments, CAS 500
3:30 pm – Seminar, CAS 502

Abstract

Molecular clouds are threaded by magnetic fields. Despite of the low ionization levels in molecular clouds, ion-neutral collisions are efficient enough in order to have the magnetic fields well coupled with the gas. However, there is still a vivid discussion on whether the magnetic field are relevant in the dynamical evolution of molecular cloud and in the process of star formation. In this talk, I will focus on the results of aperture synthesis (sub)millimeter polarimetric observations to study the role of magnetic fields in the early stages of star formation at scales from 0.1 pc (molecular dense cores) to few tens of AU. In last decade, the Submillimeter Array (SMA) have provided a handful of examples where the magnetic fields appear to play an important role in star forming molecular dense cores, showing a well organized morphology, consistent with the expected hour-glass morphology. An SMA survey of a sample of 14 massive molecular clumps have confirmed this picture. However and at least for the most massive cases, stellar feedback in more evolved regions appears to affect the net effect of magnetic fields in the dynamics of the dense cores. Finally, I will show some examples of very recently polarimetric subarcsecond observations taken with ALMA in cycle 2 and cycle 3 toward a couple star forming regions.

Thursday 3/24 Seminar: Why do we shoot rockets at the aurora?!

March 18th, 2016 in Seminars

Why do we shoot rockets at the aurora?!

Marilla Samara
GSFC

Thursday, March 24, 2016
3:00 pm – Refreshments, CAS 500
3:30 pm – Seminar, CAS 502

Abstract:
TBA

Monday 3/21 Seminar: Numerical Simulations of Black Hole Accretion

March 18th, 2016 in Seminars

Numerical Simulations of Black Hole Accretion

Prof. Ramesh Narayan
Harvard University

Monday, March 21, 2016
3:00 pm Refreshments, CAS 500
3:30 pm Seminar, CAS 502

Abstract:
TBA

Thursday 3/3 Seminar: A Fundamental Physical Process in Collisionless Heliospheric Plasmas: Interaction Between Whistler-mode Waves and Electrons

February 26th, 2016 in Seminars

A Fundamental Physical Process in Collisionless Heliospheric Plasmas: Interaction Between Whistler-mode Waves and Electrons

Wen Li
Department of Atmospheric and Oceanic Sciences
University of California, Los Angeles

Thursday, March 3, 2016
3:15 pm – Refreshments, CAS 500
3:45 pm – Seminar, CAS 502

Abstract:
In the collisionless heliospheric plasmas, wave-particle interaction is a funda- mental physical process in transferring energy and momentum between particles with different species and energies. This talk will focus on one of the im- portant wave-particle interaction processes: interaction between whistler-mode waves and electrons. Whistler-mode waves have frequencies between proton and electron cyclotron frequency and are ubiquitously present in the helio- spheric plasmas including solar wind and planetary magnetospheres. I will use Earth’s Van Allen radiation belt as “local space laboratory” to discuss the role of whistler-mode waves in electron dynamics. Van Allen radiation belt extends from 1,000 to 60,000 km above the Earth’s surface, and it is the region where highly relativistic particles reside. The extremely dynamical evolution of the radiation belt electrons is caused by various solar wind drivers and thus understanding the Sun’s influence on the radiation belt electrons is critical in forecasting space weather, which has broad impacts on our technological systems and society.

In radiation belt science, one of the most important and outstanding science questions is how tens of keV plasmasheet electrons are accelerated to ultra-relativistic energies (multiple MeV). Over the past few decades, local accelera- tion driven by whistler-mode chorus waves and inward radial diffusion have been proposed to be important to drive efficient radiation belt electron acceleration. However, the quantitative role of each physical process has not been clearly identified yet. In this talk, I will show how I use realistic global distribution of whistler-mode chorus waves obtained from an innovative technique to simulate the dynamical electron evolution during the largest geomagnetic storm over the past decade, and determine the primary electron acceleration mechanism by comparing against the Van Allen Probes electron observation. I further evaluate solar wind drivers leading to ultra-relativistic electron acceleration in the Earth’s radiation belt, which is critical in predicting space weather using the upstream solar wind conditions. At last, I will discuss my other ongoing projects and future research plans followed by unprecedented opportunities of exploring heliophysics in the future.

Monday 2/29 Seminar: The Journey of High-Energy Photons in Blazar Jets

February 26th, 2016 in Seminars

The Journey of High-Energy Photons in Blazar Jets

Lorenzo Sironi
Harvard Smithsonian Center for Astrophysics

Monday, February 29, 2016
3:15 pm – Refreshments, CAS 500
3:30 pm – Seminar, CAS 502

Abstract:

We investigate the origin and the fate of high-energy photons in blazar jets, by means of analytical theory and first-principles particle-in-cell (PIC) kinetic simulations. In magnetically-dominated jets, magnetic reconnection is often invoked as a mechanism to transfer the jet magnetic energy to the emitting particles, thus powering the observed non-thermal emission. With 2D and 3D PIC simulations, we show that magnetic reconnection in blazar jets sat- isfies all the basic conditions for the emission: extended non-thermal parti- cle distributions (with power-law slope between -2 and -1), efficient dissipa- tion and rough equipartition between particles and magnetic field in the emitting region.

TeV photons emitted by the highest energy electrons accelerated by recon- nection will interact in the intergalactic medium (IGM) with the extragalactic background light, producing a dilute beam of ultra-relativistic pairs. It is a matter of recent debate whether the energy of the pair beam is lost due to inverse Compton scattering off the CMB — resulting in ~10-100 GeV pho- tons — or heats the IGM via collective plasma instabilities. The astrophysical stakes are very high because of the large amount of energy and extensive cosmic volume involved in this process. We study the relaxation of blazar- induced beams in the IGM, by means of 2D and 3D PIC simulations. We find that at most 10% of the beam energy is deposited into the IGM plasma, so that at least 90% of the beam energy will be ultimately re-processed in the multi-GeV band.

Thursday 2/25 Seminar: MHD modeling of Heliospheric plasma environments

February 19th, 2016 in Seminars

MHD modeling of Heliospheric plasma environments

Slava Merkin
Johns Hopkins

Thursday, February 25, 2016
3:00 pm – Refreshments, CAS 500
3:45 pm – Seminar, CAS 502

Abstract:

TBD

Monday 2/22 Seminar: Solar wind-magentosphere-ionosphere coupling from auroral observations

February 19th, 2016 in Seminars

Solar wind-magentosphere-ionosphere coupling from auroral observations

Toshi Nishimura
Department of Atmospheric and Oceanic Sciences
University of California, Los Angeles

Monday, February 22, 2016
3:00 pm – Refreshments, CAS 500
3:30 pm – Seminar, CAS 502

Abstract:

Aurora, optical emissions in the upper polar atmosphere, reflects a variety of space phenomena around the Earth, and thus optical observations can be used for remote sensing of space environment. Many types of those are strongly controlled by the solar wind. This talk will introduce how the space environment around the Earth responds to different types of solar wind driving, including interplanetary shocks, discontinuities and foreshock phenomena. Of particular interest are coupling processes between structured solar wind and transient energy input into the Earth’s magnetosphere and ionosphere, which have recently been recognized as a major contributor to plasma transport. I will present our resent imaging results during those solar wind drivings, and a modeling effort for examining global coupling of such transient phenomena directly driven by structures in the solar wind. Finally, I will discuss future research directions along this subject, including Heliophysics system observatory for community-wide space-ground coordination, and imaging network.

Thursday 2/18 Seminar: Electric Fields on the Sun: How Can We Determine Them and Why Should We Care

February 12th, 2016 in Seminars

Electric Fields on the Sun: How Can We Determine Them and Why Should We Care

Maria Kazachenko
UC Berkeley / SSL

Thursday, February 18, 2016
3:00 – Refreshments, CAS 500
3:45 – Seminar, CAS 502

Abstract:
TBD

Tuesday 2/16 Seminar: TBD

February 12th, 2016 in Seminars

Topic: TBD

Atish Kamble
CfA

Tuesday, February 16, 2016
3:00 – Refreshments, CAS 500
3:45 – Seminar, CAS 502

Abstract:
TBD