This paper reports on a cavity haloscope search for dark matter axions in the galactic halo in the mass range 2.81-3.31 µeV . This search excludes the full range of axion-photon coupling values predicted in benchmark models of the invisible axion that solve the strong CP problem of quantum chromodynamics, and marks the first time a haloscope search has been able to search for axions at mode crossings using an alternate cavity configuration. Unprecedented sensitivity in this higher mass range is achieved by deploying an ultra low-noise Josephson parametric amplifier as the first-stage signal amplifier.Axions are a hypothesized particle that emerged as a result of the Peccei-Quinn solution to the strong CP problem [1][2][3]. In addition, axions are a leading darkmatter candidate that could explain 100% of the darkmatter in the Universe [4][5][6][7][8]. There are a number of mechanisms for the production of dark-matter axions in the early Universe [5,6,9,10]. For the case where U PQ (1) becomes spontaneously broken after inflation, cosmological constraints suggest an axion mass on the scale of 1 µeV or greater [11][12][13][14][15][16]. Two benchmark models for the axion are the Kim-Shifman-Vainshtein-Zakharov (KSVZ) [17,18] and Dine-Fischler-Srednicki-Zhitnitsky (DFSZ) [19,20] models. Of the two, the DFSZ model is especially compelling because of its grand unification properties [19].The Axion Dark Matter eXperiment (ADMX) searches for dark-matter axions using an axion haloscope [21], which consists of a microwave resonant cavity inside a magnetic field. In the presence of an external magnetic field, axions inside the cavity can convert to photons with frequency f = E/h, where E is the total energy of the axion, including the axion rest mass energy, plus a small kinetic energy contribution. The power expected from the conversion of an axion into microwave photons in the ADMX experiment is extremely low, O(10 −23 W ), requiring the use of a dilution refrigerator and an ultra low-noise microwave receiver to detect the photons.In limits set in a previous paper, ADMX became the only axion haloscope to achieve sensitivity to both benchmark axion models for axion masses between 2.66 and 2.81 µeV [22]. This paper reports on recent operations which extend the search for axions at DFSZ sensitivity to 2.66-3.31 µeV .The ADMX experiment consists of a 136-liter cylindrical copper-plated cavity placed in a 7.6-T field produced by a superconducting solenoid magnet. The magnet and cavity configuration are similar to the configuration described in Ref. [23,24]. A magnetic field-free region above the cavity is maintained by a counter-wound bucking magnet above the cavity. Field sensitive receiver components, such as a Josephson parametric amplifier (JPA) and circulators, are located there, and the JPA is protected by additional passive magnetic shielding.The resonant frequency of the cavity is set by two copper tuning rods that run parallel to the axis of the cavity and can be positioned between near the center of the cavity and the...

Quasi-constant heating at the footpoints of loops leads to evaporation and condensation cycles of the plasma: thermal non-equilibrium (TNE). This phenomenon is believed to play a role in the formation of prominences and coronal rain. However, it is often discarded to be involved in the heating of warm loops as the models do not reproduce observations. Recent simulations have shown that these inconsistencies with observations may be due to oversimplifications of the geometries of the models. In addition, our recent observations reveal that long-period intensity pulsations (several hours) are common in solar coronal loops. These periods are consistent with those expected from TNE. The aim of this paper is to derive characteristic physical properties of the plasma for some of these events to test the potential role of TNE in loop heating. We analyzed three events in detail using the six EUV coronal channels of SDO/AIA. We performed both a Differential Emission Measure (DEM) and a time-lag analysis, including a new method to isolate the relevant signal from the foreground and background emission. For the three events, the DEM undergoes long-period pulsations, which is a signature of periodic heating even though the loops are captured in their cooling phase, as is the bulk of the active regions. We link long-period intensity pulsations to new signatures of loop heating with strong evidence for evaporation and condensation cycles. We thus witness simultaneously widespread cooling and TNE. Finally, we discuss the implications of our new observations for both static and impulsive heating models.

Using Fourier and wavelet analysis, we critically re-assess the significance of our detection of periodic pulsations in coronal loops. We show that the proper identification of the frequency dependence and statistical properties of the different components of the power spectra provies a strong argument against the common practice of data detrending, which tends to produce spurious detections around the cut-off frequency of the filter. In addition, the white and red noise models built into the widely used wavelet code of Torrence & Compo cannot, in most cases, adequately represent the power spectra of coronal time series, thus also possibly causing false positives. Both effects suggest that several reports of periodic phenomena should be re-examined. The Torrence & Compo code nonetheless effectively computes rigorous confidence levels if provided with pertinent models of mean power spectra, and we describe the appropriate manner in which to call its core routines. We recall the meaning of the default confidence levels output from the code, and we propose new Monte-Carlo-derived levels that take into account the total number of degrees of freedom in the wavelet spectra. These improvements allow us to confirm that the power peaks that we detected have a very low probability of being caused by noise.

Long-period EUV pulsations, recently discovered to be common in active regions, are understood to be the coronal manifestation of thermal nonequilibrium (TNE). The active regions previously studied with EIT/Solar and Heliospheric Observatory and AIA/SDO indicated that long-period intensity pulsations are localized in only one or two loop bundles. The basic idea of this study is to understand why. For this purpose, we tested the response of different loop systems, using different magnetic configurations, to different stratifications and strengths of the heating. We present an extensive parameter-space study using 1D hydrodynamic simulations (1020 in total) and conclude that the occurrence of TNE requires specific combinations of parameters. Our study shows that the TNE cycles are confined to specific ranges in parameter space. This naturally explains why only some loops undergo constant periodic pulsations over several days: since the loop geometry and the heating properties generally vary from one loop to another in an active region, only the ones in which these parameters are compatible exhibit TNE cycles. Furthermore, these parameters (heating and geometry) are likely to vary significantly over the duration of a cycle, which potentially limits the possibilities of periodic behavior. This study also confirms that long-period intensity pulsations and coronal rain are two aspects of the same phenomenon: both phenomena can occur for similar heating conditions and can appear simultaneously in the simulations.

We report on the detection (10 σ) of 917 events of long-period (3 to 16 hours) intensity pulsations in the 19.5 nm passband of the SOHO Extreme ultraviolet Imaging Telescope. The data set spans from January 1997 to July 2010, i.e the entire solar cycle 23 and the beginning of cycle 24. The events can last for up to six days and have relative amplitudes up to 100%. About half of the events (54%) are found to happen in active regions, and 50% of these have been visually associated with coronal loops. The remaining 46% are localized in the quiet Sun. We performed a comprehensive analysis of the possible instrumental artifacts and we conclude that the observed signal is of solar origin. We discuss several scenarios which could explain the main characteristics of the active region events. The long periods and the amplitudes observed rule out any explanation in terms of magnetohydrodynamic waves. Thermal nonequilibrium could produce the right periods, but it fails to explain all the observed properties of coronal loops and the spatial coherence of the events. We propose that moderate temporal variations of the heating term in the energy equation, so as to avoid a thermal nonequilibrium state, could be sufficient to explain those long-period intensity pulsations. The large number of detections suggests that these pulsations are common in active regions. This would imply that the measurement of their properties could provide new constraints on the heating mechanisms of coronal loops.

In solar coronal loops, thermal non-equilibrium (TNE) is a phenomenon that can occur when the heating is both highly-stratified and quasi-constant. Unambiguous observational identification of TNE would thus permit to strongly constrain heating scenarios. Up to now, while TNE is the standard interpretation of coronal rain, the long-term periodic evolution predicted by simulations has never been observed yet. However, the detection of long-period intensity pulsations (periods of several hours) has been recently reported with SoHO/EIT, and this phenomenon appears to be very common in loops. Moreover, the three intensity-pulsation events that we recently studied with SDO/AIA show strong evidence for TNE in warm loops. In the present paper, a realistic loop geometry from LFFF extrapolations is used as input to 1D hydrodynamic simulations. Our simulations show that for the present loop geometry, the heating has to be asymmetrical to produce TNE. We analyse in detail one particular simulation that reproduces the average thermal behavior of one of the pulsating loop bundle observed with AIA. We compare the properties of this simulation with the properties deduced from the observations. The magnetic topology of the LFFF extrapolations points to the presence of sites of preferred reconnection at one footpoint, supporting the presence of asymmetric heating. In addition, we can reproduce the temporal large-scale intensity properties of the pulsating loops. This simulation further strengthens the interpretation of the observed pulsations as signatures of TNE. This thus gives important information on the heating localization and time scale for these loops.

A self‐consistent solution of the equation of wave generation and particle diffusion is obtained for gyroresonant interactions between ELF waves (a few hundred hertz) and medium‐energy electrons (a few tens of kev). A dynamic equilibrium is considered in which new particles are continuously injected and ultimately lost in the atmosphere through diffusion inside the loss cone; meanwhile, waves are generated continuously, but only a fraction of them is reflected by the ionosphere. By an iterative process the wave spectrum can be computed in shape as well as in amplitude; the distribution function of the trapped particles is also obtained. Some of the concepts that were introduced by Kennel and Petschek (1966) are more precisely defined, and attention is drawn to possible misapplications of these concepts. In particular, it is established that the so‐called limiting flux does not have a unique value and that it in fact depends upon the source strength: enhancements by factors of up to 20 can be achieved in very disturbed conditions (Kp ≥ 6). It is also demonstrated that the particle distribution function, in an equilibrium configuration, does not depend on the cold plasma density, a conclusion that is of great geophysical importance. A comparison with experimental data shows a reasonable agreement with the theory and establishes its interest for interpreting ELF hiss and equilibrium particle distributions, at least inside the plasmasphere.

Simultaneous measurements of whistler mode wave spectra in the frequency range 200 Hz to 3 kHz and of electrons in the energy range 15‐300 keV obtained on board the satellite GEOS 1 or GEOS 2, allow us to study ELF hiss generation. This study is performed both inside and outside the plasmasphere by computing the wave growth rate from the data at different magnetic latitudes λm (0 ≤ λm ≤ 30°). Inside the plasmasphere it is found that the integrated spatial gain Γ > 100 dB, is large enough to account for the observed wave intensity (∼1 pT² Hz−1) in a single transit of the wave through the equatorial region (λm ≤ 20°), which appears as the preferential amplification region. Outside the plasmasphere the situation is more intricate. The total gain Γ ranges from 16 dB to 70 dB, and the observed wave intensities are often larger than inside the plasmasphere; they could be obtained by 2‐3 transits of the waves through the equatorial region with reflection at high latitudes. This does not exclude a contribution due to amplification of exohiss leaking out of the plasmasphere at high latitudes. Both inside and outside the plasmasphere, the patterns considered to explain hiss generation which imply small values of the angle θ between the wave normal and the static magnetic field (i.e. θ < 30°) within a limited latitude range around the magnetic equator (λm ≲ 10°), are well supported by recent studies of the propagation characteristics of ELF hiss observed on board GEOS 1.