45 research outputs found
Cluster observations of non-time-continuous magnetosonic waves
Equatorial magnetosonic waves are normally observed as temporally
continuous sets of emissions lasting from minutes to hours. Recent
observations, however, have shown that this is not always the case. Using
Cluster data, this study identifies two distinct forms of these non-temporallycontinuous
emissions. The first, referred to as rising tone emissions, are characterised
by the systematic onset of wave activity at increasing proton gyroharmonic
frequencies. Sets of harmonic emissions (emission elements) are
observed to occur periodically in the region ±10◦ off the geomagnetic equator.
The sweep rate of these emissions maximises at the geomagnetic equator.
In addition, the ellipticity and propagation direction also change systematically
as Cluster crosses the geomagnetic equator. It is shown that the
observed frequency sweep rate is unlikely to result from the sideband instability
related to nonlinear trapping of suprathermal protons in the wave field.
The second form of emissions is characterised by the simultaneous onset of
activity across a range of harmonic frequencies. These waves are observed
at irregular intervals. Their occurrence correlates with changes in the spacecraft
potential, a measurement that is used as a proxy for electron densit
Multi‐parameter chorus and plasmaspheric hiss wave models
The resonant interaction of energetic particles with plasma waves, such as chorus and plasmaspheric hiss waves, plays a direct and crucial role in the acceleration and loss of radiation belt electrons that ultimately affect the dynamics of the radiation belts. In this study, we use the comprehensive wave data measurements made by the Electric and Magnetic Field Instrument Suite and Integrated Science instruments on board the two Van Allen probes, to develop multi‐parameter statistical chorus and plasmaspheric hiss wave models. The models of chorus and plasmaspheric hiss waves are presented as a function of combined geomagnetic activity (AE), solar wind velocity (V), and southward interplanetary magnetic field (Bs). The relatively smooth wave models reveal new features. Despite, the coupling between geomagnetic and solar wind parameters, the results show that each parameter still carries a sufficient amount of unique information to more accurately constrain the chorus and plasmaspheric hiss wave intensities. The new wave models presented here highlight the importance of multi‐parameter wave models, and can improve radiation belt modeling
Traveling foreshocks and transient foreshock phenomena
We use the multispacecraft capabilities of the Cluster and Time History of Events and Macroscale Interactions during Substorms (THEMIS) missions to show that two types of foreshock may be detected in spacecraft data. One is the global foreshock that appears upstream of the Earth's quasi-parallel bow shock under steady or variable interplanetary magnetic field. Another type is a traveling foreshock that is bounded by two rotational discontinuities in the interplanetary magnetic field and propagates along the bow shock. Foreshock compressional boundaries are found at the edges of both types of foreshock. We show that isolated foreshock cavities are a subset of the traveling foreshocks that form when two bounding rotational discontinuities are so close that the ultralow-frequency waves do not develop in the region between them. We also report observations of a spontaneous hot flow anomaly inside a traveling foreshock. This means that other phenomena, such as foreshock cavitons, may also exist inside this type of foreshock. In the second part of this work we present statistical properties of phenomena related to the foreshock, namely, foreshock cavities, cavitons, spontaneous hot flow anomalies, and foreshock compressional boundaries. We show that spontaneous hot flow anomalies are the most depleted transient structures in terms of the B field and plasma density inside them and that the foreshock compressional boundaries and foreshock cavities are closely related structures
Properties of Magnetic Reconnection and FTEs on the Dayside Magnetopause With and Without Positive IMF B-x Component During Southward IMF
This paper describes properties and behavior of magnetic reconnection and flux transfer events (FTEs) on the dayside magnetopause using the global hybrid-Vlasov code Vlasiator. We investigate two simulation runs with and without a sunward (positive)B-x component of the interplanetary magnetic field (IMF) when the IMF is southward. The runs are two-dimensional in real space in the noon-midnight meridional (polar) plane and three-dimensional in velocity space. Solar wind input parameters are identical in the two simulations with the exception that the IMF is purely southward in one but tilted 45 degrees toward the Sun in the other. In the purely southward case (i.e., without B-x) the magnitude of the magnetos heath magnetic field component tangential to the magnetopause is larger than in the run with a sunward tilt. This is because the shock normal is perpendicular to the IMF at the equatorial plane, whereas in the other run the shock configuration is oblique and a smaller fraction of the total IMF strength is compressed at the shock crossing. Hence, the measured average and maximum reconnection rate are larger in the purely southward run. The run with tilted IMF also exhibits a north-south asymmetry in the tangential magnetic field caused by the different angle between the IMF and the bow shock normal north and south of the equator. Greater north-south asymmetries are seen in the FTE occurrence rate, size, and velocity as well; FTEs moving toward the Southern Hemisphere are larger in size and observed less frequently than FTEs in the Northern Hemisphere.Peer reviewe
Origins of the Ambient Solar Wind: Implications for Space Weather
The Sun's outer atmosphere is heated to temperatures of millions of degrees,
and solar plasma flows out into interplanetary space at supersonic speeds. This
paper reviews our current understanding of these interrelated problems: coronal
heating and the acceleration of the ambient solar wind. We also discuss where
the community stands in its ability to forecast how variations in the solar
wind (i.e., fast and slow wind streams) impact the Earth. Although the last few
decades have seen significant progress in observations and modeling, we still
do not have a complete understanding of the relevant physical processes, nor do
we have a quantitatively precise census of which coronal structures contribute
to specific types of solar wind. Fast streams are known to be connected to the
central regions of large coronal holes. Slow streams, however, appear to come
from a wide range of sources, including streamers, pseudostreamers, coronal
loops, active regions, and coronal hole boundaries. Complicating our
understanding even more is the fact that processes such as turbulence,
stream-stream interactions, and Coulomb collisions can make it difficult to
unambiguously map a parcel measured at 1 AU back down to its coronal source. We
also review recent progress -- in theoretical modeling, observational data
analysis, and forecasting techniques that sit at the interface between data and
theory -- that gives us hope that the above problems are indeed solvable.Comment: Accepted for publication in Space Science Reviews. Special issue
connected with a 2016 ISSI workshop on "The Scientific Foundations of Space
Weather." 44 pages, 9 figure
The Earth: Plasma Sources, Losses, and Transport Processes
This paper reviews the state of knowledge concerning the source of magnetospheric plasma at Earth. Source of plasma, its acceleration and transport throughout the system, its consequences on system dynamics, and its loss are all discussed. Both observational and modeling advances since the last time this subject was covered in detail (Hultqvist et al., Magnetospheric Plasma Sources and Losses, 1999) are addressed
Ground-based and additional science support for SMILE
The joint European Space Agency and Chinese Academy of Sciences Solar wind Magnetosphere Ionosphere Link Explorer (SMILE) mission will explore global dynamics of the magnetosphere under varying solar wind and interplanetary magnetic field conditions, and simultaneously monitor the auroral response of the Northern Hemisphere ionosphere. Combining these large-scale responses with medium and fine-scale measurements at a variety of cadences by additional ground-based and space-based instruments will enable a much greater scientific impact beyond the original goals of the SMILE mission. Here, we describe current community efforts to prepare for SMILE, and the benefits and context various experiments that have explicitly expressed support for SMILE can offer. A dedicated group of international scientists representing many different experiment types and geographical locations, the Ground-based and Additional Science Working Group, is facilitating these efforts. Preparations include constructing an online SMILE Data Fusion Facility, the discussion of particular or special modes for experiments such as coherent and incoherent scatter radar, and the consideration of particular observing strategies and spacecraft conjunctions. We anticipate growing interest and community engagement with the SMILE mission, and we welcome novel ideas and insights from the solar-terrestrial community