Localization of wave fields in lower hybrid cavities

International audienceWe investigate lower hybrid wave trapping in cylindrically symmetric density depletions in the electrostatic approximation. Our investigation is inspired by previous observations of such trapping by spacecraft in the auroral region at altitudes up to about 2000km, and the recent discovery of this phenomenon at altitudes above 20000km in the inner magnetosphere. No particular shape is assumed for the density depletion, which need not be strictly zero outside some value of the radial coordinate r. Important previously known properties concerning parabolic density depletions extending to finite r are shown to hold also for arbitrary shapes and infinite extent: for a given parallel wave number kz, modes below the ambient lower hybrid frequency fLH are trapped in the density depletion (in the sense that they are evanescent outside the cavity), have a discrete spectrum and rotate in a left-handed sense, while there is a continuous spectrum of freely propagating right-handed rotating modes above fLH. New results are such that even though the density depletion may go to zero slowly with increasing r, and thus be essentially infinite in extent, there is a maximum distance within which a trapped mode with given kz and azimuthal mode number m may propagate. Furthermore, we find that for any monotonic density cavity and given kz, there is a local relation between plasma density gradient and the lowest possible frequency that can be trapped. We combine our theoretical results with spacecraft observations to find an upper bound on kz. Our examples indicate that the length of the cavities is larger than the width by a factor of at least 100

Observations of lower hybrid cavities in the inner magnetosphere by the Cluster and Viking satellites

International audienceObservations by the Viking and Cluster satellites at altitudes up to 35000km show that Lower Hybrid Cavities (LHCs) are common in the inner magnetosphere. LHCs are density depletions filled with waves in the lower hybrid frequency range. The LHCs have, until recently, only been found at altitudes up to 2000km. Statistics of the locations and general shape of the LHCs is performed to obtain an overview of some of their properties. In total, we have observed 166 LHCs on Viking during 27h of data, and 535 LHCs on Cluster during 87h of data. These LHCs are found at invariant latitudes from the auroral region to the plasmapause. A comparison with lower altitude observations shows that the LHC occurrence frequency does not scale with the flux tube radius, so that the LHCs are moderately rarer at high altitudes. This indicates that the individual LHCs do not reach from the ionosphere to 35000km altitude, which gives an upper bound for their length. The width of the LHCs perpendicular to the geomagnetic field at high altitudes is a few times the ion gyroradius, consistent with observations at low altitudes. The estimated depth of the density depletions vary with altitude, being larger at altitudes of 20000-35000km (Cluster, 10-20%), smaller around 1500-13000km (Viking and previous Freja results, a few percent) and again larger around 1000km (previous sounding rocket observations, 10-20%). The LHCs in the inner magnetosphere are situated in regions with background electrostatic hiss in the lower hybrid frequency range, consistent with investigations at low altitudes. Individual LHCs observed at high altitudes are stable at least on time scales of 0.2s (about the ion gyro period), which is consistent with previous results at lower altitudes, and observations by the four Cluster satellites show that the occurrence of LHCs in a region in space is a stable phenomenon, at least on time scales of an hour

On the Origin of Fluctuations in the Cusp Diamagnetic Cavity

We have analyzed Cluster magnetic field and plasma data during high‐altitude cusp crossing on 14 February 2003. Cluster encountered a diamagnetic cavity (DMC) during northward interplanetary magnetic field (IMF) conditions, and as IMF rotated southward, the spacecraft reencountered the cavity more at the sunward side. The DMC is characterized by a high level of magnetic field fluctuations and high‐energy electrons and protons. Ultralow‐frequency turbulence has been suggested as a mechanism to accelerate particles in DMC. We demonstrate in this paper for the first time that many of the low‐frequency fluctuations in the cavity are back and forth motion of the DMC boundaries over the spacecraft and transient reconnection signatures. We also find examples of some isolated high‐amplitude waves that could possibly be nonlinear kinetic magnetosonic modes. The lack of strong wave power at the vicinity of local ion cyclotron frequency in the DMC suggests that perhaps a mechanism other than wave‐particle heating is a dominant source for ion heating in DMCs

Stochasticity and order: studies of keratinocyte proliferation

A central tenet of stem cell biology has been that proliferating tissues are maintained through a cellular hierarchy comprising of self-renewing stem cells at the apex, multiple lineage-restricted short-lived progenitor cells, and post-mitotic differentiated cells. The wide range of colony sizes in cultured human keratinocytes has been taken to support this hypothesis. Contrary to this model, researchers using genetic lineage tracing in mouse epidermis have inferred a single progenitor population for homeostasis, and a quiescent stem cell population activated upon wounding or genetic mutation. To study the proliferative behaviour of human keratinocytes, I used live imaging in vitro at single cell resolution. This shows two modes of proliferation: Type 1 cell division is stochastic with equal odds of generating dividing or non-dividing progeny, while Type 2 cell division predominantly produces two dividing daughters. These two modes are sufficient to explain the entire range of colony sizes seen after 7-12 days of culture and does not require a spectrum of proliferative ability. This insight provides a simple way to study the effects of external factors on cell fate. To exemplify this, I observed the effects of epidermal growth factor (EGF) and the Wnt agonist R-spondin on proliferation. Here I find proliferation in type 2 colonies changes by changing the proportion of cells dividing. This has implications for the limited success of EGF therapies in clinical trials following burns. To examine clonal contributions to wound repair, I used the mouse oesophageal epithelium which is exclusively composed of, and maintained by, a single progenitor population. I developed a micro-endoscopic wounding technique that produced localised superficial wounds. Here, I found that these wounds healed by uniform contribution from surrounding keratinocytes, demonstrating that reserve stem cells are not obligatory for wound repair. In summary, my work shows that human keratinocytes in vitro have two, and only two, modes of proliferation: a stochastic mode that is insensitive to external EGF signalling, and a EGF-sensitive exponential mode. Additionally, proliferation during wound repair can occur with stochastically dividing progenitors, and does not obligate stem cell recruitment in vivo

Inferring neutral winds in the ionospheric transition region from atmospheric-gravity-wave traveling-ionospheric-disturbance (AGW-TID) observations with the EISCAT VHF radar and the Nordic Meteor Radar Cluster

Atmospheric gravity waves and traveling ionospheric disturbances can be observed in the neutral atmosphere and the ionosphere at a wide range of spatial and temporal scales. Especially at medium scales, these oscillations are often not resolved in general circulation models and are parameterized. We show that ionospheric disturbances forced by upward-propagating atmospheric gravity waves can be simultaneously observed with the EISCAT very high frequency incoherent scatter radar and the Nordic Meteor Radar Cluster. From combined multi-static measurements, both vertical and horizontal wave parameters can be determined by applying a specially developed Fourier filter analysis method. This method is demonstrated using the example of a strongly pronounced wave mode that occurred during the EISCAT experiment on 7 July 2020. Leveraging the developed technique, we show that the wave characteristics of traveling ionospheric disturbances are notably impacted by the fall transition of the mesosphere and lower thermosphere. We also demonstrate the application of using the determined wave parameters to infer the thermospheric neutral wind velocities. Applying the dissipative anelastic gravity wave dispersion relation, we obtain vertical wind profiles in the lower thermosphere.</p

The origin of semidiurnal neutral wind oscillations in the high-latitude ionospheric dynamo region

Tidal neutral wind oscillations in the high latitude ionospheric dynamo/transition region can be either in situ forced or propagate there from lower atmospheric layers. Investigating the complex mixing of tidal modes allows to determine the solar, geomagnetic and atmospheric impact on the transition region dynamics. In classical tidal theory, semidiurnal tides forced by UV and infrared absorption in lower atmospheric regions propagate upwards and are the dominant tidal mode up to about 120 km. Above that, diurnal tidal modes forced in situ by EUV absorption and ion drag due to the polar plasma convection are assumed to be dominant. We analyze a 22 day long measurement campaign with the EISCAT UHF incoherent scatter radar during September 2005. The beam-swinging experiment allows to obtain neutral winds from 96 - 142 km altitude which are combined with simultaneous meteor radar measurements. An Adaptive Spectral Filtering technique is applied to determine tidal amplitudes and phases. The zonal wind showed the expected transition from semidiurnal to diurnal oscillations at about 120 km. The meridional wind showed a more complex tidal structuring with dominant 12h oscillations below 110 km and above 130 km. General Circulation Model runs with different forcing settings are analyzed to determine the origin of these high altitude semidiurnal oscillations. The measured asymmetry of tidal amplitudes in zonal and meridional winds is found in all investigated model runs. It is shown that atmospheric tides have no influence on tidal oscillations above 120 km. Polar ion convection and EUV absorption both appear to contribute to the observed strong semidiurnal oscillations above 130 km

Determining the origin of tidal oscillations in the ionospheric transition region with EISCAT radar and global simulation data

At high-latitudes, diurnal and semidiurnal variations of temperature and neutral wind velocity can originate both in the lower atmosphere (UV or infrared absorption) or in the thermosphere-ionosphere (ion convection, EUV absorption). Determining the relative impact of different forcing mechanisms gives insight to the vertical coupling in the ionosphere. We analyse measurements from the incoherent scatter radar (ISR) facility operated by the EISCAT Scientific Association. They are complemented by meteor radar data and compared to global circulation models. The amplitudes and phases of tidal oscillations are determined by an Adaptive Spectral Filter (ASF). Measurements indicate the existence of strong semidiurnal oscillations in a two-band structure at altitudes ≲ 110 km and ≳ 130 km, respectively. Analysis of several model runs with different input settings suggest the upper band to be forced in situ while the lower band corresponds to upward-propagating tides from the lower atmosphere. This indicates the existence of an unexpectedly strong, in situ forcing mechanism for semidiurnal oscillations in the high-latitude thermosphere. It is shown that the actual transition of tides in the altitude region between 90 and 150 km is more complex than described so far