Space Physics Seminar - spring-2015
Connecting Observations of Solar Eruptions to Their Physical Underpinnings
April 3, 2015
3:30 p.m. - 4:50 p.m.
Geology 6704
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Understanding how solar eruptions form and evolve in the solar atmosphere is a key topic in solar and heliospheric physics. The nature of these eruptions, also known as Coronal Mass Ejections (CMEs), is interesting not only from a basic scientific perspective, but also because CMEs routinely disturb the inner heliosphere, and their buffeting of the Earth can sometimes have important geomagnetic consequences. Although observations and models of CMEs have improved considerably in the space age, the complexity of these events makes it surprisingly difficult to unambiguously connect various observed aspects to their physical underpinnings.
In this talk I will give an overview of present techniques and challenges involved in realistically modeling the magnetized solar corona and CMEs, with a particular focus on new methods that can help bridge the sometimes large gap between theory and observations. One such example is the study of large-scale coronal waves launched by CMEs, where using simulations as digital laboratories has aided in interpreting previously ambiguous observations. Other related aspects, such as capturing the magnetic interaction of CMEs with pre-existing coronal structures and the vetting of coronal models in general will also be discussed.
Reconnection Turbulence in Strong Guide Fields and Application to the Solar Corona
April 10, 2015
3:30 p.m. - 4:50 p.m.
Geology 6704
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The mechanism of coronal heating has been a long-standing mystery in space physics. Here we demonstrate that magnetic reconnection turbulence and the associated collisionless heating is able to produce heating rates comparable with observational constraints. To accurately describe reconnection in coronal plasmas, simulation techniques from fusion research are used that are able to efficiently capture the relevant physics in cases where the magnetic guide field far exceeds the reconnecting field in magnitude. Aside from plasma heating, consequences of flux rope mergers for nanoflare observations are discussed, as is the impact of pressure gradients on reconnection. In the latter context, a new plasma instability is presented that relies on self-reinforcing coupling between electrostatic and magnetic drifts.
Global Magnetic Topology and Large-Scale Dynamics of the Solar Corona
April 17, 2015
3:30 p.m. - 4:50 p.m.
Geology 6704
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We consider the global topology of the coronal magnetic field in relation to the large-scale dynamics of the solar corona. Our consideration includes recent results on the structural analysis of this field determined in two different approximations, namely, potential field source surface model and solar wind magnetohydrodynamic model. We identify similarities and differences between structural features of the magnetic field obtained in these two models and discuss their implications for understanding various large-scale phenomena in the solar corona.
The underlying magnetic topology manifests itself in a variety of observed morphological features such as streamers, pseudo-streamers or unipolar streamers, EUV dimmings, flare ribbons, coronal holes, and jets. For each of them, the related magnetic configuration has specific structural features, whose presence has to be not only identified but also verified on its independence from the used field model in order to reliably predict the impact of such features on physical processes in the corona. Among them are magnetic null points and minima, bald patches, separatrix surfaces and quasi-separatrix layers, and open and closed separator field lines. These features form a structural skeleton of the coronal magnetic field and are directly involved through the ubiquitous process of magnetic
reconnection in many solar dynamic phenomena such as coronal mass ejections, solar wind, acceleration and transport of energetic particles. We will pinpoint and elucidate in our overview some of such involvements that have recently received a considerable attention in our ongoing projects at Predictive Science.
Formation and Evolution of Solar Filaments and Coronal Pseudostreamers
April 24, 2015
3:30 p.m. - 4:50 p.m.
Geology 6704
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The solar dynamo and plasma convection produce three main observed structures extending from the solar surface into the corona – active regions, solar filaments (prominences when observed at the limb) and coronal holes. Each of these three key features is interlinked with the other two in its evolution and dynamics. Active regions can form clusters of magnetic activity and sunspots. When active regions decay, solar filaments form at their boundaries separating opposite magnetic polarities (neutral lines). Alternatively, decaying active regions in the presence of flux imbalance can give birth to lower latitude coronal holes. Accumulation of magnetic flux at coronal hole boundaries also creates the conditions for filament formation. Polar crown filaments are permanently present at the boundaries of the polar coronal holes. Middle-latitude and equatorial coronal holes - the result of active region evolution - can create pseudostreamers if other coronal holes of the same polarity are present. The pseudostreamer base at photospheric heights is multipolar; often one observes tripolar magnetic configurations with two neutral lines - where filaments can form - separating the coronal holes. There is debate as to the speed and nature of the wind from pseudostreamers: it could be fast, slow, or in between. I will demonstrate how the resulting wind type depends on the presence or absence of filaments at the pseudostreamer base. I will discuss mechanisms of formation and evolution for solar filaments and pseudostreamers, and the role active regions play in these processes.
Empirical Modeling the Evolution of 3D plasma and magnetic field structures
May 1, 2015
3:30 p.m. - 4:50 p.m.
Geology 6704
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Accurate evaluation of the physical processes during the substorm growth phase, including formation of field-aligned currents (FACs), isotropization by current sheet scattering, instabilities, and ionosphere-magnetosphere connection relies on knowing the realistic 3 dimensional (3D) magnetic field configuration, which cannot be reliably provided by current available empirical models. We have first developed a 2D empirical plasma sheet pressure model using the Support Vector Regression Machine (SVRM) with observational data from THEMIS and Geotail. The model predicts the plasma sheet pressure accurately with median errors of 5%, and predicted pressure gradients agree reasonably well with observed gradients obtained from two-probe measurements. Then we established a 3D substorm growth phase magnetic field model, which is uniquely constructed from empirical plasma sheet pressures under the constraint of force balance. We investigated the evolution of model pressure and magnetic field responding to increasing energy loading, and their configurations under different solar wind dynamic pressure (PSW) and sunspot number. Our model reproduces the typical growth phase evolution signatures: plasma pressure increases, magnetic field lines become more stretched, westward perpendicular current is intensified and moves earthward, and the Region-2 FACs are enhanced. The model magnetic fields agree quantitatively well with observed fields. The magnetic field is substantially more stretched under higher PSW while the dependence on sunspot number is non-linear and less substantial. By applying our modeling to a substorm event, we found that (1) the equatorward movement of proton aurora during the growth phase is mainly due to continuous stretching of magnetic field lines, (2) the ballooning instability is more favorable during late growth phase around midnight tail where there is a localized plasma beta peak, and (3) the equatorial mapping of the breakup auroral arc is at X ~ –14 RE near midnight, coinciding with the location of the maximum growth rate for the ballooning instability.
Correlation and scaling properties of non-stationary EUV intensity fluctuations in the corona
May 8, 2015
3:30 p.m. - 4:50 p.m.
Geology 6704
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A possible mechanism for heating the solar corona is via impulsive bursts or “nanoflares” The bursts might be the result of reconnection events among braided magnetic fields or possibly due to the dissipation of magnetohydrodynamic turbulence inside the loop structures. We will present evidence of nanoflare type processes using EUV coronal observations with Atmospheric Imaging Assembly (AIA) on board the Solar Dynamics Observatory (SDO). We investigate intensity variations and energy deposition in five coronal loops in non-flaring active region (AR) cores. These were selected for their strong variability in the AIA/SDO 94 Å intensity channel. Isolating the hot Fe XVIII and Fe XXI components of the 94 Å and 131 Å signals, we find that the loop apex intensity, the temperature, and electron density indicate an impulsive heating process compatible with high intensity nanoflare storms characterized by a progressive cooling pattern with the hot channels leading the emission. To account for possible non-stationary properties of intensity fluctuations we apply the method of Detrended Fluctuation Analysis (DFA). This technique allows the calculation of a scaling exponent that characterizes the correlation properties of the signal and which can be related both to the spectral and the Hurst exponents. A cross over time appears with scaling properties differing for short and long time scales. In the AR core exponents indicate a process with positive correlation which can correspond to fractional Brownian motion at long time scales. Qualitative differences exist between the exponents of the hotter and the cooler channels. In areas of diffuse emission and for all the spectral channels the time series of intensity fluctuations tend to 1/f scaling for long time scales. We show that the properties of the data can be reproduced with a physically motivated model for impulsive heating with added Gaussian noise.
Andøya Space Center - The Cost-effective Entrance to Space
May 15, 2015
3:30 p.m. - 4:50 p.m.
Geology 6704
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Andøya Space Center (ASC) was established in 1962 and provides sounding rocket, balloon and RPA (Remotely Piloted Aerial System) operations from Norway, and is host to a large array of ground based scientific instruments. ASC also owns and operates the ALOMAR lidar observatory located on the mountain Ramnan, 380 meters above sea level. ASC customers and partners include ESA, NASA, JAXA, CNES, DLR as well as a many of universities and research institutions worldwide. As a limited company, ASC is owned 90% by the Ministry of Trade, Industry and Fisheries, and 10% by Kongsberg Defence System. ASC is an ISO 9001:2008 certified company. The company’s budget for 2014 was about NOK 100 million. The number of employees is 68. The recently upgraded ASC headquarters is located on the island Andøya, two degrees north of the Arctic Circle, in the midst of Northern Norway with good connections to the mainland by plane, sea and land based transport. ASC has two launch sites for sounding rocket operations: - Andøya, Norway: N 69°17' E 16°01' - Ny-Ålesund, Svalbard: N 78°55', E 11°51' From these sites, ASC can offer a variety of rocket trajectories covering a wide range in latitude and longitude. This, together with the numerous ground based observations sites in the region, provides a great flexibility for the scientists in selecting types of phenomena to be studied. The huge impact area in the Norwegian Sea allows wide limits for rocket impact dispersion. Vehicles have been launched to an apogee of approximately 1400 km, with impact of fourth stage north of 85 degrees latitude, more than 1800 km downrange. Andøya Space Center has seven launch pads in the launch area. There are three universal launchers - U1, U3 and ATHENA, and can, if required launch rockets simultaneously. The U3 and ATHENA launchers are mounted inside heated shelters. Launch pads are also available for launchers provided by ASC users. The launch facility in Ny-Ålesund, Svalbard has one launchpad equipped with a universal launcher mounted inside a heated shelter. ASC and NAMMO Raufoss are working together to develop a new series of scientific rockets. The North Star family will consist of 3 configurations, North Star 1, North Star 2 and the North Star Launch Vehicle (NSLV), all using hybrid propulsion technology. Hybrid rocket motors have several advantages compared to solid fuel motors. A hybrid motor does not contain explosives, which means it is easier to transport. Hybrids are environmentally friendly and the liquid oxidizer is not toxic. Initially, the hybrid motors will power the proposed North Star sounding rockets 1 and 2, both carrying the ASC developed Hotel Payload. After gaining experience on the sounding rockets, the motors will be used on the proposed three stage NSLV which will be a fully hybrid powered vehicle for nanosatellites and small microsatellites up to 20 kg, launched into Polar LEO of maximum 350 km altitude from ASC about 2018. The NS1 1st stage – Nucleus, a ~28 kN motor is scheduled for its first test launch from Andøya Space Center in September 2015. The NS1 2nd stage – Aurora, will be based on 4 clustered Nuclues motors, and test launch is scheduled to 2016 from ASC. ASC offers guest offices and modern facilities for conferences and workshops. The Science Center (SC) in the main building provides the scientists with real time information from ground-based and satellite observatories. From SC the Project Scientist can determine the optimum scientific launch conditions and communicate efficiently with his team and with Launch Control.
New insights into the properties and evolution of solar wind electrons
May 22, 2015
3:30 p.m. - 4:50 p.m.
Geology 6704
Presented By:
- Chadi Salem - UC Berkeley SSL
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We present a comprehensive statistical analysis of solar wind electrons using the electron analyzers of the 3D-Plasma instrument on the Wind spacecraft. This work uses a sophisticated algorithm developed to analyze separately the different populations - core, halo and strahl - of the electron velocity distribution function (eVDF) up to 'super-halo' energies (2 keV). The code determines their respective set of parameters through fits to the measured eVDF, taking properly into account spacecraft charging and other instrumental effects. We use here several years (half a solar cycle, approximately 1.5 million of independent measurements) of core, halo and strahl electron parameters to investigate the properties of these different populations and the physical processe(s) that likely act to control and regulate them.
We discuss recent results obtained on (1) the electron temperature anisotropies and their variation with collisions and/or solar wind fluctuations and instabilities, (2) the properties of core and halo drifts in the solar wind proton frame, (3) the electron heat flux, and (4) the electron strahl. These new observations emphasize the non-negligible role of Coulomb collisions in shaping the electron distribution function and regulating of the thermal and supra thermal electrons, but that the solar wind electron expansion and compression are limited fundamentally by some instabilities under certain conditions. We also discuss the role of solar wind electrons in the microphysics and evolution of the solar wind.
From the Heliosphere, to Galaxy Clusters, Gamma Ray Bubbles Pulsars and Black Holes
May 29, 2015
3:30 p.m. - 4:50 p.m.
Geology 6704
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The in situ exploration of the Heliosphere has revealed the existence of unexpected kinds of plasmas and magnetic field configurations around the most distant planets (e.g. Uranus and Neptune) for which reliable theories can be formulated and be of help to envision the environments of a variety of recently discovered exo-planets. On larger scales, radically different kinds of plasmas have been found: in particular plasmas with “temperatures in the tens of keV are observed to be the main visible component of Galaxy Clusters, while ?-ray emitting plasma structures (“bubbles”) have been seen to emerge from the disk of Our Galaxy with dimensions of the same order as those characterizing the Galaxy. Although the theory of the plasmas that can surround pulsars has a long history, the fact that the plasmas on the surface of pulsars can have inhomogeneous features (such as hot spots) and dynamics has gained attention recently. Given the very high magnetic fields involved, the role of the electron thermal conductivity anisotropy is shown to be an important factor in these. There are important issues to be resolved in order to envision the plasmas that can surround black holes. These involve the structures that can form, such as rings and 3D spirals, the kind of transport of angular momentum that is needed to allow plasma accretion on the black hole, etc. In fact, laboratory experiments on high-energy plasmas have cast new light on basic processes that include the nature of angular momentum transport not described by a diffusion equation, magnetic reconnection events and associated production of high-energy particles, self-organization processes, etc.
Spontaneous Reconnection Onset in the Magnetotail: Wagging the Dog
June 5, 2015
3:30 p.m. - 4:50 p.m.
Geology 6704
Presented By:
- Mikhail Sitnov - JHU APL
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Bursty reconnection in the Earth¹s magnetotail has important features that distinguish it from similar processes in other space plasma regions and the simplest theoretical models. First, its possibility has long been questioned because of the stabilizing effect of electrons magnetized by the north-south (Bz) magnetic field component. Second, reconnection in the tail competes with non-reconnection ballooning/interchange and flapping motions. Third, it is distinguished by the so-called dipolarization fronts, kinetic-scale shock-like plasma structures which dominate the energy conversion. We discuss the kinetic and MHD theory of the magnetotail reconnection onset to identify the equilibria for which the spontaneous reconnection is possible. It shows that a necessary condition for the reconnection onset is a tailward gradient of the Bz field, which is indeed observed in the late substorm growth phase. 3D PIC simulations show that, when such an instability develops, it starts from the generation of an earthward plasma bulk flow with a dipolarization front at its leading edge and is followed by a magnetic topology change. These processes are accompanied by interchange and flapping motions that result in structuring of the tail activity in the local time but do not destroy the global reconnection-dominated picture. Simulations show that, in contrast to simple models, the energy conversion in the tail is largely provided by dipolarization fronts and the corresponding ion and electron temperature increases are consistent with THEMIS observations. We discuss virtual satellite observations, which reveal the potential of the MMS observations in resolving the primary plasma modes associated with the magnetotail reconnection. We also discuss the role of the dipole magnetic field in the tail structure and dynamics.