The X‐ray and gamma ray radiation from astrophysical transient sources, like X‐ray bursts from soft gamma repeaters (SGRs) and gamma ray bursts (GRBs), can affect the plasma properties of the lower ...ionosphere and middle atmosphere. Multiple very low frequency (VLF) receivers in South America, with an unprecedented high time resolution of 20 ms, detected one such series of bursts from SGR J1550‐5418 on 22 January 2009. Due to lack of other suitable means of observation corresponding to the lower part of Earth's ionosphere (∼60–100 km), the VLF detection and analysis of transient ionizing events (mostly of solar origin) has emerged as an excellent method to investigate various chemical and plasma characteristics at these heights. Extragalactic events, like SGR bursts and GRBs, with sharp modulation in their radiation time profile and very high energy photon abundances provide most unique opportunities of such studies with the possibility of extending even lower heights in the atmosphere. Here, for the first time, an extensive computer model, consisting of the combination of Monte Carlo ionization rate computation, a one dimensional atmospheric chemistry module, and VLF waveguide mode calculation, for the reconstruction of VLF signal modulation produced by SGR X‐ray burst starting from the observed spectrum and lightcurve of the event is presented. We gain some valuable insight on the nature of chemical and dynamic evolution over the entire height range of the atmosphere examined from the exercise.
Key Points
Influence of SGR X‐ray bursts on Earth's ionosphere and middle atmospheric plasma properties is examined
Extensive modeling of VLF signal modulation due to a complex SGR X‐ray burst is performed
GEANT4 Monte Carlo simulation and ion chemistry model are employed in numerical reconstruction of observed signal modulation
The daytime lower ionosphere behaves as a solar X‐ray flare detector, which can be monitored using very low frequency (VLF) radio waves that propagate inside the Earth‐ionosphere waveguide. In this ...paper, we infer the lower ionosphere sensitivity variation over a complete solar cycle by using the minimum X‐ray fluence (FXmin) necessary to produce a disturbance of the quiescent ionospheric conductivity. FXmin is the photon energy flux integrated over the time interval from the start of a solar X‐ray flare to the beginning of the ionospheric disturbance recorded as amplitude deviation of the VLF signal. FXmin is computed for ionospheric disturbances that occurred in the time interval of December–January from 2007 to 2016 (solar cycle 24). The computation of FXmin uses the X‐ray flux in the wavelength band below 0.2 nm and the amplitude of VLF signals transmitted from France (HWU), Turkey (TBB), and U.S. (NAA), which were recorded in Brazil, Finland, and Peru. The main result of this study is that the long‐term variation of FXmin is correlated with the level of solar activity, having FXmin values in the range (1 − 12) × 10−7 J/m2. Our result suggests that FXmin is anticorrelated with the lower ionosphere sensitivity, confirming that the long‐term variation of the ionospheric sensitivity is anticorrelated with the level of solar activity. This result is important to identify the minimum X‐ray fluence that an external source of ionization must overcome in order to produce a measurable ionospheric disturbance during daytime.
Key Points
Daytime lower ionosphere sensitivity to solar X-flares inferred from VLF measurements using a parameter called the minimum X‐ray fluence
The minimum X‐ray fluence value shows no dependence with the size of solar flares
Long‐term minimum X‐ray fluence variation correlates with the level of solar Lyman α flux and anticorrelates with the ionospheric sensitivity
Measurements of the potential gradient (PG) under fair weather conditions at new locations are crucial for monitoring the global electric circuit. In this study, we analyzed the variations in PG ...recorded in Ica city, Peru, during the period from March 2018 to December 2022. Our analysis involved a detailed comparison of PG with various meteorological parameters such as rainfall, wind speed and direction, dust storms, dust devils and fog to establish the characteristic PG curve under fair weather conditions (referred to as the standard curve). We identified a significant threshold of 3.5 m/s for strong winds, which helps prevent the influence of sea breeze and dust lifting on PG. Our results showed the impact of 'Paracas' dust storms on PG, which aligns with patterns observed in other arid regions worldwide. Subsequently, we calculated monthly, seasonal, and annual averages of the standard curve which showed a response likely associated with local convective processes on the PG diurnal variation. Furthermore, the seasonal variation of PG reveals higher values during June, July, and August compared to December, January, and February. These differences are attributed to seasonal changes in aerosol concentrations, potentially influenced by biomass-burning activities in Peru. Additionally, we performed a wavelet transform analysis of PG hourly values. We found the periodicities of 1 day, ∼188 days, and 360 days which are related to the diurnal, semiannual and annual periods. During the months between January and March 2018 was found an intense period of ∼45-day likely associated with one of the strongest Madden-Julian Oscillation events. Through this comprehensive investigation, we deepen our understanding of the intricate relationships among meteorological conditions, sea breeze, dust storms, and the PG in arid regions like Ica.
•PG diurnal and seasonal variation are related to local convection and biomass-burning at Ica, respectively.•Sea breeze and ‘Paracas’ winds have clear signatures in the PG diurnal variation.•A 45-day period related to the Madden-Jullian Oscillation has been found.
The almost unexplored frequency window from submillimeter to mid-infrared (mid-IR) may bring new clues about the particle acceleration and transport processes and the atmospheric thermal response ...during solar flares. Because of its technical complexity and the special atmospheric environment needed, observations at these frequencies are very sparse. The High Altitude THz Solar Photometer (HATS) is a full-Sun ground-based telescope designed to observe the continuum from the submillimeter to the mid-IR. It has a 457-mm spherical mirror with the sensor in its primary focus. The sensor is a Golay cell with high sensitivity in a very wide frequency range. The telescope has a polar mount, and a custom-built data acquisition system based on a 32 ksamples per second, 24 bits (72 dB dynamic range), 8 channels analog-to-digital board. Changing only the composition of the low- and band-pass filters in front of the Golay cell, the telescope can be setup to detect very different frequency bands; making the instrument very versatile. In this article we describe the telescope characteristics and its development status. Moreover, we give estimates of the expected fluxes during flares.
Using soft X‐ray measurements from detectors onboard the Geostationary Operational Environmental Satellite (GOES) and simultaneous high‐cadence Lyman‐α observations from the Large Yield Radiometer ...(LYRA) onboard the Project for On‐Board Autonomy 2 (PROBA2) ESA spacecraft, we study the response of the lower part of the ionosphere, the D region, to seven moderate to medium‐size solar flares that occurred in February and March of 2010. The ionospheric disturbances are analyzed by monitoring the resulting sub‐ionospheric wave propagation anomalies detected by the South America Very Low Frequency (VLF) Network (SAVNET). We find that the ionospheric disturbances, which are characterized by changes of the VLF wave phase, do not depend on the presence of Lyman‐α radiation excesses during the flares. Indeed, Lyman‐α excesses associated with flares do not produce measurable phase changes. Our results are in agreement with what is expected in terms of forcing of the lower ionosphere by quiescent Lyman‐α emission along the solar activity cycle. Therefore, while phase changes using the VLF technique may be a good indicator of quiescent Lyman‐α variations along the solar cycle, they cannot be used to scale explosive Lyman‐α emission during flares.
Key Points
Excesses of flare Ly‐a emissions are below the VLF technique sensitivity
Quiescent Ly‐a can be tracked using VLF waves, transient flare excesses cannot
Observed disturbances are in the expected range of solar activity conditions
In this paper we analyse the effects of 25 solar flare events over long VLF propagation paths, one of them with the very remarkable property of crossing nearly the centre with the lowest magnetic ...field intensity in the South Atlantic Magnetic Anomaly (SAMA). The phase of the VLF transmitter signal (NPM: 21.4 kHz) on Lualualei, Hawaii, was recorded at the stations Punta Lobos (PLO, Peru) and Atibaia (ATI, Brazil) between 2007 March and 2011 September. Both paths NPM-PLO and NPM-ATI are collinear, and the comparison of the recorded phases suggests a descent of the lower ionosphere quiescent reflection height, possibly associated with the weakening of the Earth magnetic field in the SAMA region.
•Observational evidence on the effects of the Magnetic Anomaly on the quiescent ionospheric reference height.•Within the Magnetic Anomaly the reference height is lower by ~1-3 km.•Conclusion obtained from a very simple comparison of phase recorded on two parallel VLF propagation paths.
We have studied the dynamics of the nighttime lower ionosphere height through continuous monitoring of the VLF modal interference distance (so-called distance D). Since the distance D is related to ...the nighttime propagation modes within the Earth-Ionosphere waveguide, it provides information of the nighttime reflection height (hN). We have used a long-term VLF narrowband database of almost 8 years (2006–2014) from a long transequatorial VLF propagation path between the transmitter NPM (Hawaii, 21.4 kHz) and the receiver ATI (Atibaia, Brazil). Our results show that hN assumes lower values during northern hemisphere wintertime as compared with summertime. By using the Lomb-Scargle periodogram, periodicities around 180 (SAO), 365 (AO) and 800 (QBO) days have been found, being the periodicity around 180 days stronger than all other oscillations. Since these large-scale oscillations are commonly observed in several measurable parameters of the mesosphere-lower thermosphere (MLT) region, our results suggest that the nighttime lower ionosphere can be strongly influenced by the dynamics of the MLT region. The effect of the long-term solar activity on hN is also studied, resulting in high negative correlation (R = −0.91). This effect makes hN decrease around 1.2 km from low to high solar activity. This result suggests a control of the solar radiation on the nighttime lower ionosphere, and hence, on the electron density at night.
•The lower ionosphere nighttime height is estimated using the VLF modal interference.•The nighttime height shows semi-annual and annual oscillations.•The nighttime height shows negative correlation with the mesospheric temperature.•High negative correlation is found between the nighttime height and solar activity.
We report the temporal evolution of the excess brightness temperature ΔTb above solar active regions (ARs) observed with the Solar Submillimeter Telescope (SST) at 212 (λ = 1.4 mm) and 405 GHz (λ = ...0.7 mm) during Cycles 23 and 24. Comparison with the sunspot number (SSN) yields a Pearson's correlation coefficient R = 0.88 and 0.74 for 212 and 405 GHz, respectively. Moreover, when only Cycle 24 is taken into account the correlation coefficients go to 0.93 and 0.81 for each frequency. We derive the spectral index between SST frequencies and find a slight anticorrelation with the SSN (R = −0.25); however, since the amplitude of the variation is lower than the standard deviation we cannot draw a definite conclusion. Indeed, remains almost constant within the uncertainties with a median value of 0 characteristic of an optically thick thermal source. Since the origin of the AR submillimeter radiation is thermal continuum produced at chromospheric heights, the strong correlation between ΔTb and the magnetic cycle evolution could be related to the available free magnetic energy to be released in reconnection events.
We investigate the relation between sudden phase anomaly (SPA) amplitudes (Δφ) and solar X‐ray flares importance and study if it has a solar activity cycle dependence. We find a very significant ...correlation between Δφ and the X‐ray fluences, FX (time‐integrated photon fluxes) in the range 0.5–2 Å. Compared with earlier works, the improvements of the Δφ versus FX relation allow us to study separately solar events as a function of their occurrence in the solar activity cycle, and we find that the Δφ versus FX relation is different depending on the epoch within the solar cycle. In particular, a minimum X‐ray fluence of 2.5 × 10−6 J m−2 is needed during solar activity minimum to trigger a SPA, while during solar maximum, 7.0 × 10−6 J m−2 is required. Similarly, a solar flare will produce a SPA during solar minimum that is greater by ∼2.6° M m−1 than would a flare of the same size during solar maximum. These results confirm recent findings about the dependence with the solar activity of the ionospheric undisturbed D region sensitivity. A possible consequence would be the monitoring of the long‐term solar irradiance, which maintains the D region, through measurements of VLF wave propagation properties.
Pronounced amplitude minima are observed during the subionospheric propagation of VLF waves at times (Terminator Times) when the Terminator Line crosses given locations along the propagation path. ...The distance between such two successive minima is called the modal interference distance D, which is related to nighttime mode propagation in the Earth‐ionosphere waveguide. Therefore, the temporal behavior of the distance D can bring information on the dynamics of the nighttime lower ionosphere and on the presence of external forcing agents, including those associated with seismic activity. In this paper we present a methodology to estimate D based on the measure and analysis of the pronounced VLF amplitude minima. We have used a long‐term database of almost 5 years from three different VLF propagation paths from the South America VLF Network. We emphasize that the accuracy of the determination of the distance D achieved by our method is better than those obtained in earlier studies. The reason for that is the use of a long‐term continuous database, from different parallel propagation paths mainly oriented along the west‐to‐east direction. We discuss typical properties of the obtained distance D, as the simultaneous occurrence of amplitude minima for parallel propagation paths, anomalous values of D at locations where the Terminator Line is close to the receiver, and the derivation of the undisturbed nighttime ionospheric height at hN ~ 88 km.
Key Points
A methodology to estimate the VLF modal interference distance D is developed
The undisturbed height of the nighttime waveguide is deduced
Distance D can be used to monitor the nighttime height of the waveguide
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