Energy deposition in the ionosphere through a global field line resonance

International audienceWe present an interval whereby we can estimate the energy dissipation in the ionosphere through an externally-driven field line resonance. In this paper, we utilise an interval described in general by Rae et al. (2005), where the global magnetospheric cavity was shown to be energised via a high solar wind speed stream. Using the ground-based instrumentation available, we estimate the spatial extent of the generated pulsations to be at least 10° in latitude and 65° in longitude, a sizeable fraction of the dusk-sector ionosphere. Using a fortuitous conjunction with the Polar spacecraft, we compare point measurements of the net downward Poynting vector to the estimated Joule heating rate in the ionosphere, and find that model values of the Pedersen conductance are reasonable. In the interval of interest, we estimate the total dissipation rate during a global field line resonance to be comparable to that reported in substorm studies. Previous studies have estimated the total energy deposition via field line resonance to be up to 4% of that deposited during a small substorm. However, in this paper we find that the total energy deposited via Joule heating may actually be 30% or more of the energy deposited in the ionosphere during a substorm cycle using a conservative estimate of the pulsation duration

Comparison of the open‐closed separatrix in a global magnetospheric simulation with observations: The role of the ring current

Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/95344/1/jgra20421.pd

On the estimates of the ring current injection and decay

In the context of the space weather predictions, forecasting ring current strength (and of the Dst index) based on the solar wind upstream conditions is of specific interest for predicting the occurrence of geomagnetic storms. In the present paper, we have studied separately its two components: the Dst injection and decay. In particular, we have verified the validity of the Burton's equation for estimating the ring current energy balance using the equatorial electric merging field instead of the original parameter V Bs (V is the solar wind speed and Bs is the southward component of the Interplanetary Magnetic Field, IMF). Then, based on this equation, we have used the phasespace method to determine the best-fit approximations for the ring current injection and decay as functions of the equatorial merging electric field (Em). Results indicate that the interplanetary injection is statistically higher than in previous estimations using V Bs . Specifically, weak but not-null ring current injection can be observed even during northward IMF, when previous studies considered it to be always zero. Moreover, results about the ring current decay indicate that the rate of Dst decay is faster than its predictions derived by using V Bs . In addition, smaller quiet time ring current and solar wind pressure corrections are contributing to Dst estimates obtained by Em instead of V Bs . These effects are compensated, so that the statistical Dst predictions using the equatorial electric merging field or using V Bs are about equivalent

A multispacecraft analysis of a small-scale transient entrained by solar wind streams

The images taken by the Heliospheric Imagers (HIs), part of the SECCHI imaging package onboard the pair of STEREO spacecraft, provide information on the radial and latitudinal evolution of the plasma compressed inside corotating interaction regions (CIRs). A plasma density wave imaged by the HI instrument onboard STEREO-B was found to propagate towards STEREO-A, enabling a comparison between simultaneous remotesensing and in situ observations of its structure to be performed. In situ measurements made by STEREO-A show that the plasma density wave is associated with the passage of a CIR. The magnetic field compressed after the CIR stream interface (SI) is found to have a planar distribution. Minimum variance analysis of the magnetic field vectors shows that the SI is inclined at 54° to the orbital plane of the STEREO-A spacecraft. This inclination of the CIR SI is comparable to the inclination of the associated plasma density wave observed by HI. A small-scale magnetic cloud with a flux rope topology and radial extent of 0.08 AU is also embedded prior to the SI. The pitch-angle distribution of suprathermal electrons measured by the STEREO-A SWEA instrument shows that an open magnetic field topology in the cloud replaced the heliospheric current sheet locally. These observations confirm that HI observes CIRs in difference images when a small-scale transient is caught up in the compression region

Aberrant crossed corticospinal facilitation in muscles distant from a spinal cord injury.

Crossed facilitatory interactions in the corticospinal pathway are impaired in humans with chronic incomplete spinal cord injury (SCI). The extent to which crossed facilitation is affected in muscles above and below the injury remains unknown. To address this question we tested 51 patients with neurological injuries between C2-T12 and 17 age-matched healthy controls. Using transcranial magnetic stimulation we elicited motor evoked potentials (MEPs) in the resting first dorsal interosseous, biceps brachii, and tibialis anterior muscles when the contralateral side remained at rest or performed 70% of maximal voluntary contraction (MVC) into index finger abduction, elbow flexion, and ankle dorsiflexion, respectively. By testing MEPs in muscles with motoneurons located at different spinal cord segments we were able to relate the neurological level of injury to be above, at, or below the location of the motoneurons of the muscle tested. We demonstrate that in patients the size of MEPs was increased to a similar extent as in controls in muscles above the injury during 70% of MVC compared to rest. MEPs remained unchanged in muscles at and within 5 segments below the injury during 70% of MVC compared to rest. However, in muscles beyond 5 segments below the injury the size of MEPs increased similar to controls and was aberrantly high, 2-fold above controls, in muscles distant (>15 segments) from the injury. These aberrantly large MEPs were accompanied by larger F-wave amplitudes compared to controls. Thus, our findings support the view that corticospinal degeneration does not spread rostral to the lesion, and highlights the potential of caudal regions distant from an injury to facilitate residual corticospinal output after SCI

Remote sensing the magnetosphere using ground-based observations of ULF waves

Ground-based magnetometers record the signature of ultra-low frequency (ULF) wave energy incident from the magnetosphere. A portion of the signal comes from resonant ULF wave structures that form between the northern and southern ionospheres, known as field line resonances (FLRs) in the near-Earth space plasma. It is well known that the resonant frequency depends on the length, the strength, and the distribution of plasma mass along the geomagnetic field from one ionosphere to the other. Given an accurate description of the geomagnetic field in space, the ULF resonant frequencies may be used to remote sense the plasma mass density in the magnetosphere. Identifying the continuum or more directly driven resonant frequencies is the key to the remote sensing process. Techniques involving amplitude, phase, and single and multi-instrument data are discussed. The procedures developed for identifying FLRs at various locations on the Earth's surface have traditionally been accomplished using magnetometers but may also be applied to other ground-based instrumentation such as high-frequency radars, Doppler sounders and meridian scanning photometers. Data analysis methods are reviewed and the development of new analysis methods is encouraged by providing code to compute the Wavelet transform. Remote sensed estimates of plasma mass density are compared with spacecraft data, leading to a discussion of accuracy and further refinements