Polar Cap Absorption Research Articles

Comparison of modeling of the polar cap absorption effect with observations at the AARI ground-based network

An elliptical model of cutoff for solar cosmic rays (SCR) at high latitudes has been calculated for SCR events and geomagnetic storms that occurred in February, March, and September 2014, June 2015, and September 2017 in order to predict the polar cap absorption (PCA) effect. The prediction is compared with observations of the PCA effect at the AARI network of six ionosondes, located in the north of Russia in a wide range of longitudes from 30 to 170 and magnetic latitudes from 56 to 64. We examine the dependence of the cutoff latitude on the magnetic storm strength, as well as the effect of additional ionization of the dayside and nightside ionosphere by SCR protons and electrons. The model demonstrates a satisfactory statistical accuracy up to 0.83 for the PCA prediction. We discuss prospects for further improvement of the elliptical model, such as consideration of the impact of SCRs of higher energies and inclusion of the PC index of energy flux into the polar cap.

Сравнение моделирования эффекта поглощения в полярной шапке с наблюдениями на сети станций ААНИИ

An elliptical model of cutoff for solar cosmic rays (SCR) at high latitudes has been calculated for SCR events and geomagnetic storms that occurred in February, March, and September 2014, June 2015, and September 2017 in order to predict the polar cap absorption (PCA) effect. The prediction is compared with observations of the PCA effect at the AARI network of six ionosondes, located in the north of Russia in a wide range of longitudes from 30 to 170 and magnetic latitudes from 56 to 64. We examine the dependence of the cutoff latitude on the magnetic storm strength, as well as the effect of additional ionization of the dayside and nightside ionosphere by SCR protons and electrons. The model demonstrates a satisfactory statistical accuracy up to 0.83 for the PCA prediction. We discuss prospects for further improvement of the elliptical model, such as consideration of the impact of SCRs of higher energies and inclusion of the PC index of energy flux into the polar cap.

Low-frequency solar radio type II bursts and their association with space weather events during the ascending phase of solar cycle 25

Abstract. Type II solar radio bursts are signatures of the coronal shocks and, therefore, particle acceleration events in the solar atmosphere and interplanetary space. Type II bursts can serve as a proxy to provide early warnings of incoming solar storm disturbances, such as geomagnetic storms and radiation storms, which may further lead to ionospheric effects. In this article, we report the first observation of 32 type II bursts by measuring various plasma parameters that occurred between May 2021 and December 2022 in solar cycle 25. We further evaluated their accompanying space weather events in terms of ionospheric total electron content (TEC) enhancement using the rate of TEC index (ROTI). In this study, we find that at heliocentric distance ∼1–2 R⊙, the shock and the Alfvén speeds are in the range 504–1282 and 368–826 km−1, respectively. The Alfvén Mach number is of the order of 1.2≤MA≤1.8 at the above-mentioned heliocentric distance. In addition, the measured magnetic field strength is consistent with the earlier reports and follows a single power law B(r)=6.07r-3.96G. Based on the current analysis, it is found that 19 out of 32 type II bursts are associated with immediate space weather events in terms of radio blackouts and polar cap absorption events, making them strong indications of space weather disruption. The ROTI enhancements, which indicate ionospheric irregularities, strongly correlate with GOES X-ray flares, which are associated with the type II radio bursts recorded. The diurnal variability in ROTI is proportional to the strength of the associated flare class, and the corresponding longitudinal variation is attributed to the difference in longitude. This article demonstrates that since type II bursts are connected to space weather hazards, understanding various physical parameters of type II bursts helps to predict and forecast the space weather.

Occurrence rate and duration of space weather impacts on high-frequency radio communication used by aviation

High frequency (HF) radio wave propagation is sensitive to space weather-induced ionospheric disturbances that result from enhanced photoionization and energetic particle precipitation. Recognizing the potential risk to HF radio communication systems used by the aviation industry, as well as potential impacts on GNSS navigation and the risk of elevated radiation levels, the International Civil Aviation Organization (ICAO) initiated the development of a space weather advisory service. For HF systems, this service specifically identifies shortwave fadeout, auroral absorption, polar cap absorption, and post-storm maximum useable frequency depression (PSD) as phenomena impacting HF radio communication and specifies moderate and severe event thresholds to describe event severity. This paper examines the occurrence rate and duration of events crossing the moderate and severe thresholds. Shortwave fadeout was evaluated based on thresholds in the solar X-ray flux. Analysis of 40-years of solar X-ray flux data showed that moderate and severe level solar X-ray flares were observed, on average, 123 and 5 times per 11-year solar cycle, respectively. The mean event duration was 68 min for moderate level events and 132 min for severe level events. Auroral absorption events crossed the moderate threshold for 40 events per solar cycle, with a mean event duration of 5.1 h. The severe threshold was crossed for 3 events per solar cycle with a mean event duration of 12 h. Polar cap absorption had the longest mean duration at ~8 h for moderate events and 1.6 days for severe events; on average, 24 moderate and 13 severe events were observed per solar cycle. Moderate and severe thresholds for shortwave fadeout, auroral absorption, and polar cap absorption were used to determine the expected impacts on HF radio communication. Results for polar cap absorption and shortwave fadeout were consistent with each other, but the expected impact for auroral absorption was shown to be 2–3 times higher. Analysis of 22 years of ionosonde data showed moderate, and severe PSD events occurred, on average, 200 and 56 times per 11-year solar cycle, respectively. The mean event duration was 5.5 h for moderate-level events and 8.5 h for severe-level events. During solar cycles 22 and 23, HF radio communication was expected to experience moderate or severe impacts due to the ionospheric disturbances caused by space weather, a maximum of 163 and 78 days per year, respectively, due to the combined effect of absorption and PSD. The distribution of events is highly non-uniform with respect to the solar cycle: 70% of moderate or severe events were observed during solar maximum compared to solar minimum.

Model‐Based Properties of the Dayside Open/Closed Boundary: Is There a UT‐Dependent Variation?

AbstractThe open‐closed boundary (OCB) defines a region of significant transformation in Earth's protective magnetic shield. Principle among these changes is the transition of magnetic field lines from having two foot points, one in each hemisphere, to one foot point at Earth, the other mapping to the solar wind. Charged particles in the solar wind are able to follow these open field lines into Earth's upper atmosphere. The OCB also defines the polar cap boundary. Being able to identify and track the OCB allows study of several components of the geomagnetic system. Among them are the electrodynamics of the geomagnetic field and the reconnection balance between the dayside and nightside of the geomagnetic field. Furthermore, the OCB can provide insights into the precipitation of energetic protons into the ionosphere. Using the Tsyganenko model of the geomagnetic field (T96), we demonstrate a diurnal fluctuation which we call the Universal Time (UT) effect of the OCB. This UT effect is independent of all other inputs. We anticipate this UT effect to have important consequences in modeling the OCB and other polar cap‐associated structures, especially polar cap absorption events that adversely affect high‐frequency radio wave propagation in polar regions.

SuperDARN Radar‐Derived HF Radio Attenuation During the September 2017 Solar Proton Events

AbstractTwo solar proton events in September 2017 had a significant impact on the operation of the Super Dual Auroral Radar Network (SuperDARN), a global network of high‐frequency (HF) radars designed for observing F region ionospheric plasma convection. Strong polar cap absorption caused near‐total loss of radar backscatter, which prevented the primary SuperDARN data products from being determined for a period of several days. During this interval, the high‐latitude and polar cap radars measured unusually low levels of background atmospheric radio noise. We demonstrate that these background noise measurements can be used to observe the spatial and temporal evolution of the polar cap absorption region, using an approach similar to riometry. We find that the temporal evolution of the SuperDARN radar‐derived HF attenuation closely follows that of the cosmic noise absorption measured by a riometer. Attenuation of the atmospheric noise up to 10 dB at 12 MHz is measured within the northern polar cap, and up to 14 dB in the southern polar cap, which is consistent with the observed backscatter loss. Additionally, periods of enhanced attenuation lasting 2–4 hr are detected by the midlatitude radars in response to M‐ and X‐class solar flares. Our results demonstrate that SuperDARN's routine measurements of atmospheric radio noise can be used to monitor 8‐ to 20‐MHz radio attenuation from middle to polar latitudes, which may be used to supplement riometer data and also to investigate the causes of SuperDARN backscatter loss during space weather events.

Examination of the relationship between riometer-derived absorption and the integral proton flux in the context of modeling polar cap absorption

Energetic protons can penetrate into the ionosphere increasing ionization in the D-region causing polar cap absorption that may potentially block high frequency radio communications for transpolar flights. The protons are guided by the geomagnetic field into the high-latitude polar cap region. Riometers monitor variations in ionospheric absorption by observing the level of background cosmic radio noise. Current polar cap absorption modelling techniques are based on the linear relationship between absorption and the square root of the integral proton flux, which has previously only been demonstrated using data from a single high-latitude polar station. The proportionality constant describing this relationship is evaluated for two different polar cap absorption events occurring 07-11 March 2012 and 23 January 2012 to 01 February 2012. Examination of the proportionality constant using data from riometers distributed between 60° and 90° magnetic latitude reveals a previously unreported latitudinal dependence for data at magnetic latitudes of ≤66.8° on the dayside and ≤70.8° on the nightside. Incorporating the latitudinal dependence into the current D-Region Absorption Prediction (D-RAP) absorption model improves the agreement between measurement-derived and modelled parameters by increasing the correlation coefficient between data sets, reducing the root mean square error, and reducing the bias.

Improving the twilight model for polar cap absorption nowcasts

During Solar Proton Events (SPE), energetic protons ionize the polar mesosphere causing HF radiowave attenuation, more strongly on the dayside where the effective recombination coefficient, αeff, is low. Polar cap absorption (PCA) models predict the 30 MHz cosmic noise absorption, A, measured by riometers, based on real-time measurements of the integrated proton flux-energy spectrum, J. However, empirical models in common use cannot account for regional and day-to-day variations in the day- and nighttime profiles of αeff(z) or the related sensitivity parameter, m=A/√J. Large prediction errors occur during twilight when m changes rapidly, and due to errors locating the rigidity cutoff latitude. Modeling the twilight change in m as a linear or Gauss error-function transition over a range of solar-zenith angles (χl < χ < χu) provides a better fit to measurements than selecting day or night αeff profiles based on the Earth-shadow height. Optimal model parameters were determined for several polar cap riometers for large SPEs in 1998-2005. The optimal χl parameter was found to be most variable, with smaller values (as low as 60°) post-sunrise compared with pre-sunset, and with positive correlation between riometers over a wide area. Day and night values of m exhibited higher correlation for closely spaced riometers. A nowcast simulation is presented in which rigidity boundary latitude and twilight model parameters are optimized by assimilating age-weighted measurements from 25 riometers. The technique reduces model bias, and root-mean-squared errors are reduced by up to 30% compared with a model employing no riometer data assimilation.

A polar cap absorption model optimization based on the vertical ionograms analysis

Space weather events significantly affect the high frequency (HF) radio wave propagation. The now-casting and forecasting of HF radio wave absorption is important for the HF communication industries. This paper assimilates vertical sounding data into an absorption model to improve its performance as a now-casting tool. The approach is a modification of the algorithm elaborated by Sauer and Wilkinson, which is based on the riometer data. The optimization is focused on accounting for short timescale variation of the absorption. It should be noted that the expression of the frequency dependence of absorption induced by the energetic particle precipitation employed in Sauer and Wilkinson model is based on the riometer data at frequencies of 20, 30, and 50MHz. The approach suggested in this paper provides an opportunity for expanding the frequency dependence of the absorption for frequencies below 10MHz. The simulation of the vertical ionograms in the polar cap region uses a computational model designed to overcome the high frequency wave propagation problem in high latitude of the Earth. HF radio wave absorption induced by solar UV illumination, X-ray flares and energetic particles precipitation is taken into consideration in our model. The absorption caused by the energetic particle precipitation is emphasized, because the study is focused on HF wave propagation in polar cap region. A comparison of observed and simulated vertical ionograms enables the coefficients, which relate absorption (day-time and night-time) to integral proton flux to be refined. The values of these coefficients determined from evaluation of the data recorded by any reliable ionosonde are valid for absorption calculation in high-latitude region.

Influence of X-ray and polar cap absorptions on vertical and oblique sounding ionograms on different latitudes

High frequency (HF) radio band is important for the long-range communications and over-the-horizon surveillance, particularly in the polar cap region where the ground infrastructure may be limited. However, the space weather events drastically affect the high frequency radio wave propagation so that the ability to provide now-casting and forecasting of HF radio wave absorption is important for users relying on the HF communications. During the space weather events such as solar proton events and X-ray flares the high-latitude ionosphere becomes a particularly efficient absorber of HF radio waves. There is therefore a need to develop accurate HF propagation prediction services. Absorption of the HF field caused by the X-ray flux, Solar Ultra-Violet flux and proton precipitations is investigated in this paper for the event of the solar flare observed on 11 April 2013. The effects of the X-ray flux and proton precipitations on the structure of the vertical and oblique ionograms for different latitudes are estimated. The simulation of the vertical and oblique ionograms was performed on the basis of the computational model of the ionosphere oriented to applications into the high frequency wave propagation problems. The absorption effects induced by the proton precipitations and X-ray flux are calculated according to the algorithm elaborated by Sauer and Wilkinson and D-region Absorption Model (D-RAP) available from the NOAA Space Weather Prediction Center. The simulated vertical and oblique ionograms with the absorption effects taken into account and the measured ionograms exhibit a fairly good similarity.

Assimilation of real-time riometer measurements into models of 30 MHz polar cap absorption

Space weather events may adversely affect high frequency (HF) radio propagation, hence the ability to provide nowcasting and forecasting of HF radio absorption is key for industries that rely on HF communications. This paper presents methods of assimilating 30 MHz radio absorption measurements into two types of ionospheric polar cap absorption (PCA) model to improve their performance as nowcasting tools. Type 1 models calculate absorption as m times the square root of the flux of solar protons above an energy threshold, E t . Measurements from 14 riometers during 94 solar proton events (1995–2010) are assimilated by optimising the day and night values of m by linear regression. Further non-linear optimisations are demonstrated in which parameters such as E t are also optimised and additional terms characterise local time and seasonal variations. These optimisations reduce RMS errors by up to 36%. Type 2 models incorporate altitude profiles of electron and neutral densities and electron temperatures. Here the scale height of the effective recombination coefficient profile in the D-region is optimised by regression. Furthermore, two published models of the rigidity cut-off latitude (CL) are assessed by comparison with riometer measurements. A small improvement in performance is observed by introducing a 3-h lag in the geomagnetic index K p in the CL models. Assimilating data from a single riometer in the polar cap reduces RMS errors below 1 dB with less than 0.2 dB bias. However, many high-latitude riometers now provide absorption measurements in near-real time and we demonstrate how these data may be assimilated by fitting a low-order spherical harmonic function to both the measurements and a PCA model with optimised parameters.

Ultrarelativistic electrons in the near cosmos and X‐ray aurora in the middle polar atmosphere

The rare phenomenon of ultrarelativistic electron precipitation into the middle polar atmosphere, prevalent under calm geophysical conditions, was established from ground‐based radio wave measurements during the period of 1982–1992. Precipitating electrons with energy ∼ 100 MeV and sufficient density to generate X‐ and gamma‐ray bremsstrahlung create a sporadic layer of ionization in the atmosphere under the regular D layer of the ionosphere. Very low frequency radio waves reflect from this sporadic layer with abnormal weakening and with an unusually low height of reflection. The layer has a horizontal linear scale of about several thousand kilometers, with a thickness in altitude of about 20–30 km, and persists for several hours. Due to this layer of electric conductivity, the effective height of this “ground‐ionized atmosphere” waveguide diminishes in exceptional cases by 2–2.5 times. The auroras of X‐ray bremsstrahlung have been detected by the reflection of radio waves with wavelengths of 30–20 km. This phenomenon may be termed “a polar cap absorption effect of the second kind” as an electron analog of proton precipitation.

George C. Reid (1929–2011)

George Colvin Reid, a pioneering solar and climate scientist, passed away at his home in Boulder, Colo., on 6 May 2011 after a brief battle with pancreatic cancer. George was 81. His legacy includes an impressive and prolific scientific career that spanned nearly 60 years. George was a world leader in atmospheric science with an exceptionally broad understanding of atmospheric phenomena encompassing both dynamics and chemistry. Early in his career, George wrote a number of influential papers with Harold Leinbach, Gordon Little, and Colin Hines on energetic particles in solar flares and the effect of such particles on the Earth's atmosphere. He and Harold Leinbach coined the often quoted phrase “polar cap absorption” and its acronym “PCA,” which have entered the lexicon of atmospheric science to describe the interaction of solar flares on the Earth's atmosphere at high latitudes. As his career progressed, his research interests expanded to include solar energetic particles, solar influences on the upper atmosphere, ion chemistry in the upper atmosphere, noctilucent clouds, the dynamics and chemistry of the middle atmosphere, the mechanisms of global climate change with emphasis on the tropics, and the influence of solar variability on the lower atmosphere and ocean temperatures.

Elliptical model of cutoff boundaries for the solar energetic particles measured by POES satellites in December 2006

Experimental data from a constellation of five NOAA Polar Orbiting Environmental Satellites (POES), satellites were used for studying the penetration of solar energetic particles (SEP) to high latitudes during long‐lasting SEP events on 5–15 December 2006. We determined cutoff latitudes for electrons with energies &gt;100 keV and &gt;300 keV and for protons with energies from 240 keV to &gt;140 MeV. The large number of satellites allowed us to derive snapshots of the cutoff boundaries with 1‐hour time resolution. The boundaries were fitted well by ellipses. On the basis of the elliptical approach, we developed a model of cutoff latitudes for protons and electrons in the northern and southern hemispheres. The cutoff latitude is represented as a function of rigidity, R, of particles; MLT, geomagnetic indices Dst, Kp, and AE; and dipole tilt angle PS. The model predicts tailward and duskward shifting of the cutoff boundaries in relation to intensification of the cross‐tail current, field‐aligned currents, and symmetrical and asymmetrical parts of the ring current. The model was applied for prediction of polar cap absorption (PCA) effects observed at high latitudes by the Canadian Advanced Digital Ionosonde network of ionosondes. It was found that the PCA effects are related mainly to intense fluxes of &gt;2.5 MeV protons and &gt;100 keV electrons, which contribute mostly to the ionization of ionospheric D‐layer at altitudes of ∼75 to 85 km. This finding was confirmed independently by FORMOSAT‐3/COSMIC observations of the SEP‐associated enhancements of electron content at altitudes of ∼80 km.

Early Japanese contributions to space weather research (1945–1960)

Abstract. Major contributions by Japanese scientists in the period of 1945 to 1960 are reviewed. This was the period when the foundation of the space weather research was laid by ground-based observations and theoretical research. Important contributions were made on such subjects as equatorial ionosphere in quiet times, tidal wind system in the ionosphere, formation of the F2 layer, VLF propagation above the ionosphere, and precursory phenomena (type IV radio outburst and polar cap absorption) to storms. At the IGY (1957, 1958), research efforts were intensified and new programs in space and Antarctica were initiated. Japanese scientists in this discipline held a tight network for communication and collaboration that has been kept to this day.

Study on solar sources and polar cap absorption events recorded in Antarctica

Particularly intense events occurred on the Sun in a period around minimum of solar activity during cycle 23. We investigated the characteristics of September 2005 and December 2006 events and the properties of the correlated observations of ionospheric absorption, obtained by a 30 MHz riometer installed at Mario Zucchelli Station (MZS-Antarctica), and of geomagnetic activity recorded at Scott Base (Antarctica). Solar events are studied using the characteristics of CMEs measured with SoHO/LASCO coronagraphs and the temporal evolution of solar energetic protons in different energy ranges measured by GOES 11 spacecraft. Analysing these data, we have determined how these effects are finally observed on the Earth’s surface not only in the ionospheric absorption of radio waves and in the intense geomagnetic activity, but also as significant variations of cosmic ray modulation, even at high energies.

Global mapping of ionospheric HF/VHF radio wave absorption due to solar energetic protons

Simple, one‐parameter algorithms are applied to the observed energetic proton flux as provided by instruments aboard the GOES series of satellites to yield estimates of the high‐latitude HF and VHF radio wave absorption for day and night, respectively. These results are extended to full daily coverage by treating the effects of solar illumination, geomagnetic cutoff variation, and frequency dependence over the entire earth. Validation calculations of the polar cap absorption of HF radio waves have been performed for 11 larger solar energetic particle events during the period from 1992 to 2002 and the results are compared to observations of 30 MHz riometers operated by the Air Force Geophysics Laboratory and located at Thule, Greenland. Prediction of the minimum event duration from current flux level is also obtained, and a specimen presentation of the north and south polar caps illustrates the graphical output of the model for the peak of the 6 December 2006 solar proton event.

Observations of D-region structure and atmospheric tides with PFISR during active aurora

We report on combined spectral measurements of the D-region ionosphere and the polar summer mesosphere and lower thermosphere performed on 9 June 2007 with the 450 MHz Poker Flat Incoherent Scatter Radar (PFISR) under active auroral conditions. Observations during the first 7 h occurred at nighttime and revealed strong temporal correlations between enhanced auroral precipitation, the occurrence of polar mesosphere summer echoes (PMSE), and enhanced electron densities ( N e ) extending to much lower altitudes ( ∼ 60 – 80 km ) than the previously reported at these latitudes without the presence of a polar cap absorption (PCA) event. PMSE and lower D-region echoes most often occurred together and PMSE were more often visible to PFISR before the N e of the D-region was sufficiently high to be detected. These measurements also enabled definition of high precision horizontal winds (errors of 2 – 3 m s - 1 ) in the ∼ 64 – 90 km altitude range, which has previously been possible only with MF and HF radars, with the Arecibo 430 MHz IS radar (ISR), or with other ISRs during PCA events. This observing technique allowed for ∼ 24 h of almost continuous measurements of the meridional and zonal wind fields and thus permitted the measurements of atmospheric tides over this large altitude range. We determine amplitude and phase for both the diurnal and semi-diurnal tides showing the latter to be stronger as expected at these latitudes. We also present observations of a gravity wave during a quiet period indicating that PFISRs sensitivity is sufficiently high to obtain reliable spectral information even when electron densities are not enhanced by aurora or a PCA event and are as low as 10 9 e - / m 3 . These results show the capabilities of PFISR to study the neutral dynamics in the polar D-region and mesosphere and lower thermosphere (MLT) over an unprecedented altitude range.

Emergent Activities in the Arctic: A Space Weather Opportunity

Emergent Activities in the Arctic: A Space Weather Opportunity

Monitoring and Forecasting the Ionosphere Over Europe: The DIAS Project

Knowledge of the state of the upper atmosphere, and in particular its ionospheric part, is very important in several applications affected by space weather, especially the communications and navigation systems that rely on radio transmission. To better classify the ionosphere and forecast its disturbances over Europe, a data collection endeavour called the European Digital Upper Atmosphere Server (DIAS) was initiated in 2004 by a consortium formed around several European ionospheric stations that transmit in real-time ionospheric parameters automatically scaled. The DIAS project is a collaborative venture of eight institutions funded by the European Commission eContent Programme. The project seeks to improve access to digital information collected by public European institutes and to expand its use. The main objective of the DIAS project is to develop a pan-European digital data collection describing the state of the upper atmosphere, based on real-time information and historical data collections provided by most of the operating ionospheric stations in Europe. Various groups of users require data specifying upper atmospheric conditions over Europe for nowcasting and forecasting purposes. The DIAS system is designed to distribute such information. The successful operation of DIAS is based on the effective use of observational data in operational applications through the development of new added-value ionospheric products and services that best fit the needs of the market. DIAS is a unique European system, and its continuous operation will efficiently support radio propagation services with the most reliable information. DIAS began providing services to users in August 2006. The Need for Accurate Ionospheric Products Radio frequency communications and satellite positioning and navigation systems are applications most affected by ionospheric disturbances. Such disturbances can cause drastic and large-scale changes in the usable ranges of high frequency (HF) or below HF bands affecting standard ground-to-ground and submarine communication systems. The characteristics of an ionospheric propagation channel, whether it is HF or transionospheric frequencies, are highly variable on timescales ranging from a few seconds to the 11-year solar cycle. Even during its quietest periods, the Sun produces electromagnetic radiation and solar wind, both of which can affect a variety of geomagnetic and ionospheric phenomena, which in turn affect radio waves propagating through the ionosphere. Hence day-to-day and hour-to-hour changes in propagation channel characteristics can occur.