104 research outputs found
Modeling stretched solitary waves along magnetic field lines
International audienceA model is presented for a new type of fast solitary waves which is observed in downward current regions of the auroral zone. The three-dimensional, coherent structures are electrostatic, have a positive potential, and move along the magnetic field lines with speeds on the order of the electron drift. Their parallel potential profile is flattened and cannot fit to the Gaussian shape used in previous work. We develop a detailed BGK model which includes a flattened potential and an assumed cylindrical symmetry around a centric magnetic field line. The model envisions concentric shells of trapped electrons slowly drifting azimuthally while bouncing back and forth in the parallel direction. The electron dynamics is analysed in terms of three basic motions that occur on different time scales characterized by the cyclotron frequency We , the bounce frequency wb , and the azimuthal drift frequency wg. The ordering We >> wb >> wg is required. Self-consistent distribution functions are calculated in terms of approximate constants of motion. Constraints on the parameters characterizing the amplitude and shape of the stretched solitary wave are discussed
In situ spatiotemporal measurements of the detailed azimuthal substructure of the substorm current wedge
The substorm current wedge (SCW) is a fundamental component of geomagnetic substorms. Models tend to describe the SCW as a simple line current flowing into the ionosphere toward dawn and out of the ionosphere toward dusk, linked by a westward electrojet. We use multispacecraft observations from perigee passes of the Cluster 1 and 4 spacecraft during a substorm on 15 January 2010, in conjunction with ground-based observations, to examine the spatial structuring and temporal variability of the SCW. At this time, the spacecraft traveled east-west azimuthally above the auroral region. We show that the SCW has significant azimuthal substructure on scales of 100km at altitudes of 4000-7000km. We identify 26 individual current sheets in the Cluster 4 data and 34 individual current sheets in the Cluster 1 data, with Cluster 1 passing through the SCW 120-240s after Cluster 4 at 1300-2000km higher altitude. Both spacecraft observed large-scale regions of net upward and downward field-aligned current, consistent with the large-scale characteristics of the SCW, although sheets of oppositely directed currents were observed within both regions. We show that the majority of these current sheets were closely aligned to a north-south direction, in contrast to the expected east-west orientation of the preonset aurora. Comparing our results with observations of the field-aligned current associated with bursty bulk flows (BBFs), we conclude that significant questions remain for the explanation of SCW structuring by BBF-driven wedgelets. Our results therefore represent constraints on future modeling and theoretical frameworks on the generation of theSCW. Key Points The substorm current wedge (SCW) has significant azimuthal structure Current sheets within the SCW are north-south aligned The substructure of the SCW raises questions for the proposed wedgelet scenari
Quantifying the Energy Budget in the Solar Wind from 13.3-100 Solar Radii
A variety of energy sources, ranging from dynamic processes like magnetic
reconnection and waves to quasi-steady terms like the plasma pressure, may
contribute to the acceleration of the solar wind. We utilize a combination of
charged particle and magnetic field observations from the Parker Solar Probe
(PSP) to attempt to quantify the steady-state contribution of the proton
pressure, the electric potential, and the wave energy to the solar wind proton
acceleration observed by PSP between 13.3 and ~100 solar radii (RS). The proton
pressure provides a natural kinematic driver of the outflow. The ambipolar
electric potential acts to couple the electron pressure to the protons,
providing another definite proton acceleration term. Fluctuations and waves,
while inherently dynamic, can act as an additional effective steady-state
pressure term. To analyze the contributions of these terms, we utilize radial
binning of single-point PSP measurements, as well as repeated crossings of the
same stream at different distances on individual PSP orbits (i.e. "fast radial
scans"). In agreement with previous work, we find that the electric potential
contains sufficient energy to fully explain the acceleration of the slower wind
streams. On the other hand, we find that the wave pressure plays an
increasingly important role in the faster wind streams. The combination of
these terms can explain the continuing acceleration of both slow and fast wind
streams beyond 13.3 RS
High-cadence measurements of electron pitch-angle distributions from Solar Orbiter SWA-EAS burst mode operations
Circadian Preference Modulates the Neural Substrate of Conflict Processing across the Day
Human morning and evening chronotypes differ in their preferred timing for sleep and wakefulness, as well as in optimal daytime periods to cope with cognitive challenges. Recent evidence suggests that these preferences are not a simple by-product of socio-professional timing constraints, but can be driven by inter-individual differences in the expression of circadian and homeostatic sleep-wake promoting signals. Chronotypes thus constitute a unique tool to access the interplay between those processes under normally entrained day-night conditions, and to investigate how they impinge onto higher cognitive control processes. Using functional magnetic resonance imaging (fMRI), we assessed the influence of chronotype and time-of-day on conflict processing-related cerebral activity throughout a normal waking day. Sixteen morning and 15 evening types were recorded at two individually adapted time points (1.5 versus 10.5 hours spent awake) while performing the Stroop paradigm. Results show that interference-related hemodynamic responses are maintained or even increased in evening types from the subjective morning to the subjective evening in a set of brain areas playing a pivotal role in successful inhibitory functioning, whereas they decreased in morning types under the same conditions. Furthermore, during the evening hours, activity in a posterior hypothalamic region putatively involved in sleep-wake regulation correlated in a chronotype-specific manner with slow wave activity at the beginning of the night, an index of accumulated homeostatic sleep pressure. These results shed light into the cerebral mechanisms underlying inter-individual differences of higher-order cognitive state maintenance under normally entrained day-night conditions
Front Neurol
Objective: Recent research suggests that sleep disorders or changes in sleep stages or EEG waveform precede over time the onset of the clinical signs of pathological cognitive impairment (e.g., Alzheimer's disease). The aim of this study was to identify biomarkers based on EEG power values and spindle characteristics during sleep that occur in the early stages of mild cognitive impairment (MCI) in older adults. Methods: This study was a case-control cross-sectional study with 1-year follow-up of cases. Patients with isolated subjective cognitive complaints (SCC) or MCI were recruited in the Bordeaux Memory Clinic (MEMENTO cohort). Cognitively normal controls were recruited. All participants were recorded with two successive polysomnography 1 year apart. Delta, theta, and sigma absolute spectral power and spindle characteristics (frequency, density, and amplitude) were analyzed from purified EEG during NREM and REM sleep periods during the entire second night. Results: Twenty-nine patients (8 males, age = 71 +/- 7 years) and 29 controls were recruited at T0. Logistic regression analyses demonstrated that age-related cognitive impairment were associated with a reduced delta power (odds ratio (OR) 0.072, P < 0.05), theta power (OR 0.018, P < 0.01), sigma power (OR 0.033, P < 0.05), and spindle maximal amplitude (OR 0.002, P < 0.05) during NREM sleep. Variables were adjusted on age, gender, body mass index, educational level, and medication use. Seventeen patients were evaluated at 1-year follow-up. Correlations showed that changes in self-reported sleep complaints, sleep consolidation, and spindle characteristics (spectral power, maximal amplitude, duration, and frequency) were associated with cognitive impairment (P < 0.05). Conclusion: A reduction in slow-wave, theta and sigma activities, and a modification in spindle characteristics during NREM sleep are associated very early with a greater risk of the occurrence of cognitive impairment. Poor sleep consolidation, lower amplitude, and faster frequency of spindles may be early sleep biomarkers of worsening cognitive decline in older adults
Electrons in the Young Solar Wind: First Results from the Parker Solar Probe
The Solar Wind Electrons Alphas and Protons experiment on the Parker Solar
Probe (PSP) mission measures the three-dimensional electron velocity
distribution function. We derive the parameters of the core, halo, and strahl
populations utilizing a combination of fitting to model distributions and
numerical integration for electron distributions measured near
the Sun on the first two PSP orbits, which reached heliocentric distances as
small as AU. As expected, the electron core density and temperature
increase with decreasing heliocentric distance, while the ratio of electron
thermal pressure to magnetic pressure () decreases. These quantities
have radial scaling consistent with previous observations farther from the Sun,
with superposed variations associated with different solar wind streams. The
density in the strahl also increases; however, the density of the halo plateaus
and even decreases at perihelion, leading to a large strahl/halo ratio near the
Sun. As at greater heliocentric distances, the core has a sunward drift
relative to the proton frame, which balances the current carried by the strahl,
satisfying the zero-current condition necessary to maintain quasi-neutrality.
Many characteristics of the electron distributions near perihelion have trends
with solar wind flow speed, , and/or collisional age. Near the Sun,
some trends not clearly seen at 1 AU become apparent, including
anti-correlations between wind speed and both electron temperature and heat
flux. These trends help us understand the mechanisms that shape the solar wind
electron distributions at an early stage of their evolution
Exploring Solar-Terrestrial Interactions via Multiple Observers (A White Paper for the Voyage 2050 long-term plan in the ESA Science Programme)
This paper addresses the fundamental science question: "How does solar wind
energy flow through the Earth's magnetosphere, how is it converted and
distributed?". We need to understand how the Sun creates the heliosphere, and
how the planets interact with the solar wind and its magnetic field, not just
as a matter of scientific curiosity, but to address a clear and pressing
practical problem: space weather, which can influence the performance and
reliability of our technological systems, in space and on the ground, and can
endanger human life and health.
Much knowledge has already been acquired over the past decades, but the
infant stage of space weather forecasting demonstrates that we still have a
vast amount of learning to do. We can tackle this issue in two ways: 1) By
using multiple spacecraft measuring conditions in situ in the magnetosphere in
order to make sense of the fundamental small scale processes that enable
transport and coupling, or 2) By taking a global approach to observations of
the conditions that prevail throughout geospace in order to quantify the global
effects of external drivers.
A global approach is now being taken by a number of space missions under
development and the first tantalising results of their exploration will be
available in the next decade. Here we propose the next step-up in the quest for
a complete understanding of how the Sun gives rise to and controls the Earth's
plasma environment: a tomographic imaging approach comprising two spacecraft
which enable global imaging of magnetopause and cusps, auroral regions,
plasmasphere and ring current, alongside in situ measurements. Such a mission
is going to be crucial on the way to achieve scientific closure on the question
of solar-terrestrial interactions
Exploring solar-terrestrial interactions via multiple imaging observers
How does solar wind energy flow through the Earth's magnetosphere, how is it converted and distributed? is the question we want to address. We need to understand how geomagnetic storms and substorms start and grow, not just as a matter of scientific curiosity, but to address a clear and pressing practical problem: space weather, which can influence the performance and reliability of our technological systems, in space and on the ground, and can endanger human life and health. Much knowledge has already been acquired over the past decades, particularly by making use of multiple spacecraft measuring conditions in situ, but the infant stage of space weather forecasting demonstrates that we still have a vast amount of learning to do. A novel global approach is now being taken by a number of space imaging missions which are under development and the first tantalising results of their exploration will be available in the next decade. In this White Paper, submitted to ESA in response to the Voyage 2050 Call, we propose the next step in the quest for a complete understanding of how the Sun controls the Earth's plasma environment: a tomographic imaging approach comprising two spacecraft in highly inclined polar orbits, enabling global imaging of magnetopause and cusps in soft X-rays, of auroral regions in FUV, of plasmasphere and ring current in EUV and ENA (Energetic Neutral Atoms), alongside in situ measurements. Such a mission, encompassing the variety of physical processes determining the conditions of geospace, will be crucial on the way to achieving scientific closure on the question of solar-terrestrial interactions.Peer reviewe
Magnetic reconnection as a mechanism to produce multiple protonpopulations and beams locally in the solar wind
Context. Spacecraft observations early revealed frequent multiple proton populations in the solar wind. Decades of research on their origin have focused on processes such as magnetic reconnection in the low corona and wave-particle interactions in the corona and locally in the solar wind.Aims.This study aims to highlight that multiple proton populations and beams are also produced by magnetic reconnection occurring locally in the solar wind. Methods. We use high resolution Solar Orbiter proton velocity distribution function measurements, complemented by electron and magnetic field data, to analyze the association of multiple proton populations and beams with magnetic reconnection during a period of slow Alfv\'enic solar wind on 16 July 2020. Results. At least 6 reconnecting current sheets with associated multiple proton populations and beams, including a case of magnetic reconnection at a switchback boundary, are found during this day. This represents 2% of the measured distribution functions. We discuss how this proportion may be underestimated, and how it may depend on solar wind type and distance from the Sun. Conclusions. Although suggesting a likely small contribution, but which remains to be quantitatively assessed, Solar Orbiter observations show that magnetic reconnection must be considered as one of the mechanisms that produce multiple proton populations and beams locally in the solar wind
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