AE Index Research Articles

Modeling the storm time behavior of the magnetosonic waves using solar wind parameters

AbstractWe build empirical models of magnetosonic waves (also called as equatorial noise) both outside and inside the plasmapause, based on the Time History of Events and Macroscale Interactions during Substorms fast‐survey data during the period from 1 May 2010 to 30 November 2014, which utilize solar wind measurements as model inputs, i.e., interplanetary magnetic field (IMF) Bz, solar wind speed (VSW), and dynamic pressure (PSW). For model development, the time delay of magnetosonic wave amplitude relative to the solar wind parameters is first identified based on correlation analysis. We find that PSW is better correlated with the variation of wave amplitude outside the plasmapause than VSW, while the opposite is true inside the plasmapause. Among the solar wind parameters, the preceding southward IMF Bz with the time delay of 3–4 h and 4–5 h, corresponding to outside and inside the plasmapause, respectively, yields the best correlation with the variation of wave amplitude. In this study, we use the solar wind parameter‐based model to investigate magnetosonic wave activity during corotating interaction region‐driven storm. The results show that with increasing southward IMF Bz, wave activity is amplified mostly on the dayside both outside and inside the plasmapause and high amplitude is more shifted toward the prenoon sector, while increasing PSW, it is amplified only in the postnoon sector outside the plasmapause. Our statistical model can explain the observed amplitudes by 38% on average outside the plasmapause and 26% inside, and their model performance is generally higher than that of a model using AE index only as an input.

Effects of Interplanetary Shock Inclinations on Nightside Auroral Power Intensity

We derive fast forward interplanetary (IP) shock speeds and impact angles to study the geoeffectivness of 461 IP shocks that occurred from January 1995 to December 2013 using ACE and WIND spacecraft data. The geomagnetic activity is inferred from the SuperMAG project data. SuperMAG is a large chain which employs more than 300 ground stations to compute enhanced versions of the traditional geomagnetic indices. The SuperMAG auroral electroject SME index, an enhanced version of the traditional AE index, is used as an auroral power (AP) indicator. AP intensity jumps triggered by shock impacts are correlated with both shock speed and impact angle. It is found that high AP intensity events typically occur when high speed IP shocks impact the Earths magnetosphere with the shock normal almost parallel to the Sun-Earth line. This result suggests that symmetric and strong magnetospheric compression leads to favorable conditions for intense auroral power release, as shown previously by simulations and observations. Some potential mechanisms will be discussed.

Dynamic plasmapause model based on THEMIS measurements

AbstractThis paper presents a dynamic plasmapause location model established based on 5 years of Time History of Events and Macroscale Interactions during Substorms (THEMIS) measurements from 2009 to 2013. In total, 5878 plasmapause crossing events are identified, sufficiently covering all 24 magnetic local time (MLT) sectors. Based on this plasmapause crossing database, we investigate the correlations between plasmapause locations with solar wind parameters and geomagnetic indices. Input parameters for the best fits are obtained for different MLT sectors, and finally, we choose five input parameters to build a plasmapause location model, including 5 min‐averaged SYM‐H, AL, and AU indices as well as hourly‐averaged AE and Kp indices. two out‐of‐sample comparisons on the evolution of the plasmapause is shown during two magnetic storms, demonstrating good agreement between model results and observations. Two major advantages are achieved by this model. First, this model provides plasmapause locations at 24 MLT sectors, still providing good consistency with observations. Second, this model is able to reproduce dynamic variations of the plasmapause on timescales as short as 5 min.

Radiação quilométrica auroral

Vários fenômenos puderam ser mais bem estudados ou até mesmo só foram descobertos depois do desenvolvimento dos satélites e espaçonaves. Dentre esses fenômenos podemos destacar a radiação quilométrica auroral (AKR, do inglês auroral kilometric radiation). O fenômeno foi descoberto apenas há pouco mais de 40 anos; as frequências típicas são muito baixas para penetrar na ionosfera em direção à Terra. Sua geração se dá através de interações onda-partícula na região noturna do planeta, acima da ionosfera auroral, sendo a energia necessária proveniente dos feixes de elétrons que se precipitam nessas regiões. É gerada principalmente no modo extraordinário (modo-X), no intervalo de frequências de 20-800 kHz, próximo à frequência local ciclotrônica de elétrons. Sabe-se muito bem que a AKR é intensificada durante subtempestades magnéticas, sendo bem correlacionada com o índice AE. Entretanto, estudos recentes têm mostrado que a radiação desaparece nas fases inicial e principal de algumas tempestades magnéticas, apesar do grande aumento do índice AE e de correntes alinhadas ao campo. Nesses casos, a radiação volta a ser emitida durante a fase de recuperação. Esse comportamento sugere que o campo elétrico alinhado ao campo magnético que acelera os elétrons que se precipitam e induz correntes alinhadas não é formado nas fases inicial e principal de algumas tempestades magnéticas. O intuito desse artigo é apresentar aos leitores a AKR e suas características, tornando esse importante fenômeno mais conhecido entre estudantes e professores de física.

Robust statistical properties of the size of large burst events in AE

AbstractGeomagnetic indices provide a comprehensive data set with which to quantify space climate, that is, how the statistical likelihood of activity varies with the solar cycle. We characterize space climate by the AE index burst distribution. Burst sizes are constructed by thresholding the AE time series; a burst is the sum of the excess in the time series for each time interval over which the threshold is exceeded. The distribution of burst sizes is two component with a crossover in behavior at thresholds ≈1000 nT. Above this threshold, we find a range over which the mean burst size varies weakly with threshold for both solar maxima and minima. The burst size distribution of the largest events is exponential. The relative likelihood of these large events varies from one solar maximum and minimum to the next. Given the relative overall activity of a solar maximum/minimum, these results constrain the likelihood of extreme events of a given size.

Spatial distribution of magnetic fluctuation power with period 40 to 600 s in the magnetosphere observed by THEMIS

AbstractUltralow frequency (ULF) fluctuations are ubiquitous in the magnetosphere and have significant influence on the energetic particle transport. We use Time History of Events and Macroscale Interactions during Substorms (THEMIS) data to give the spatial distribution of the Pi2/Pc4 and Pc5 band magnetic fluctuation amplitude near the magnetic equator in the magnetosphere. Statistical results can be summarized as follows: (1) strong ULF fluctuations are common in the magnetotail plasma sheet; the amplitude of all three components of magnetic fluctuations decreases with decreasing radial distance; (2) during periods of high AE index, fluctuations can propagate toward the Earth as far as the data cutoff in the nightside of the magnetosphere, and the amplitude of magnetic fluctuations is clearly stronger near the dusk sector of the synchronous orbit than that near the dawn sector, suggesting that the substorm particle injection has significant contribution to these fluctuations; (3) intense compressional Pc5 band magnetic fluctuations are a persistent feature near two flanks of the magnetosphere. Clear peaks of the compressional Pi2/Pc4 band magnetic fluctuation power near two flanks can be found during periods of fast solar wind, while the power of compressional Pi2/Pc4 band fluctuations is weak when the solar wind is slow. (4) Solar wind dynamic pressure and its variations can globally affect the ULF fluctuation power in the magnetosphere. Magnetic fluctuations near the noonside can penetrate from the magnetopause to the synchronous orbit or inner when solar wind pressure variations are large.

The relationship between plasmapause, solar wind and geomagnetic activity between 2007 and 2011

Abstract. Taking advantage of the Cluster satellite mission and especially the observations made by the instrument WHISPER to deduce the electron number density along the orbit of the satellites, we studied the relationships between the plasmapause positions (LPP) and the following LPP indicators: (a) solar wind coupling functions Bz (Z component of the interplanetary magnetic field vector, B, in GSM system), BV (related to the interplanetary electric field; B is the magnitude of the interplanetary magnetic field vector, V is solar wind velocity), and dΦmp/dt (which combines different physical processes responsible for the magnetospheric activity) and (b) geomagnetic indices Dst, Ap and AE. The analysis is performed separately for three magnetic local time (MLT) sectors (Sector1 – night sector (01:00–07:00 MLT); Sector2 – day sector (07:00–16:00 MLT); Sector3 – evening sector (16:00–01:00 MLT)) and for all MLTs taken together. All LPP indicators suggest the faster plasmapause response in the postmidnight sector. Delays in the plasmapause responses (hereafter time lags) are approximately 2–27 h, always increasing from Sector1 to Sector3. The obtained fits clearly resolve the MLT structures. The variability in the plasmapause is the largest for low values of LPP indicators, especially in Sector2. At low activity levels,LPP exhibits the largest values on the dayside (in Sector2) and the smallest on the postmidnight side (Sector1). Displacements towards larger values on the evening side (Sector3) and towards lower values on the dayside (Sector2) are identified for enhanced magnetic activity. Our results contribute to constraining the physical mechanisms involved in the plasmapause formation and to further study the still not well understood related issues.

Statistical analysis of extreme auroral electrojet indices

Extreme auroral electrojet activities can damage electrical power grids due to large induced currents in the Earth, degrade radio communications and navigation systems due to the ionospheric disturbances and cause polar-orbiting satellite anomalies due to the enhanced auroral electron precipitation. Statistical estimation of extreme auroral electrojet activities is an important factor in space weather research. For this estimation, we utilize extreme value theory (EVT), which focuses on the statistical behavior in the tail of a distribution. As a measure of auroral electrojet activities, auroral electrojet indices AL, AU, and AE, are used, which describe the maximum current strength of the westward and eastward auroral electrojets and the sum of the two oppositely directed in the auroral latitude ionosphere, respectively. We provide statistical evidence for finite upper limits to AL and AU and estimate the annual expected number and probable intensity of their extreme events. We detect two different types of extreme AE events; therefore, application of the appropriate EVT analysis to AE is difficult.

Investigation into the linear relationship between the AE, Dst and ap indices during different magnetic and solar activity conditions

There are numerous geomagnetic indices used in monitoring various magnetospheric and ionospheric phenomena. Some of the most widely used indices are the ap, AE and Dst. In this work, the relationship between these three geomagnetic indices is investigated at different levels of solar and magnetic activity. 3-h average data spanning 8-years were used—high (HSA), moderate (MSA) and low solar activity (LSA) periods cover the years 1999–2001, 2004–2005, and 2006, 2009–2010 respectively. All the investigated correlation pairs recorded the highest/lowest during the LSA/HSA periods. The ap/AE correlation was found to be highest ranging within 70–78 % at any solar activity. The ap versus AE and Dst multiple correlation reached 94.0, 92.1, and 89.2 % for HSA, MSA, and LSA conditions, respectively, and 72.1, 83.3, and 80.0 % for the main phase, recovery phase and quiet conditions respectively. Moreover, higher percentage correlations were observed for the ap/AE pair at any geomagnetic conditions than for the ap/Dst and AE/Dst pairs. The ring current index Dst is observed to have a greater influence on ap during geomagnetic storm periods.

Ionospheric variation during pulsating aurora

AbstractWe have statistically analyzed data from the European Incoherent Scatter (EISCAT) UHF/VHF radars in Tromsø (69.60°N, 19.20°E), Norway, to reveal how the occurrence of pulsating auroras (PsAs) modifies the electron density profile in the ionosphere. By checking five winter seasons' (2007–2012) observations of all‐sky aurora cameras of the National Institute of Polar Research in Tromsø, we have extracted 21 cases of PsA. During these PsA events, either the UHF or VHF radar of EISCAT was operative and the electron density profiles were obtained along the field‐aligned or vertical direction near the zenith. From these electron density measurements, we calculated hmE (E region peak height) and NmE (E region peak density), which are proxies for the energy and flux of the precipitating PsA electrons, respectively. Then, we examined how these two parameters changed during the evolution of 21 PsA events in a statistical fashion. The results can be summarized as follows: (1) hmE is lower (the energy of precipitation electrons is higher) during the periods of PsA than that in the surrounding interval; (2) when NmE is higher (flux of PsA electrons is larger), hmE tends to be lower (precipitation is harder); (3) hmE is lower and NmE is larger in the later magnetic local time; and (4) when the AE index during the preceding substorm is larger, hmE is lower and NmE is larger. These tendencies are discussed in terms of the characteristics of particles and plasma waves in the source of PsA in the magnetosphere. In addition to the statistics of the EISCAT data, we carried out several detailed case studies, in which the altitude profiles of the electron density were derived by separating the On and Off phases of PsA. This allows us to estimate the true altitude profiles of the PsA ionization, which can be used for estimating the characteristic energy of the PsA electrons and better understanding the wave‐particle interaction process in the magnetosphere.

Association of ionospheric storms and substorms of Global Electron Content with proxy AE index

Storm time modeling of Global Electron Content (GEC) calculated from GIM-TEC for 1999–2013 is associated with new proxy of Auroral Electrojet variability expressed as a smoothed and normalized Auroral Electrojet index (AEsn). The variability in GEC is captured by the computation of DGEC which is obtained by taking the hourly ratio of instant GEC to median of GEC values at the same hour of 7 preceding days. The storm onset is determined by a joint analysis of variations in IMF-B magnitude, its derivative (dB/dt) and direction of IMF-Bz together with sudden increase in AE exceeding 900nT. The start of the pre-storm period is chosen to be 7h prior to the storm onset time and the storm recovery period ends 41h after the storm onset. The hourly AEsn is related to DGEC during the storm period through a polynomial whose coefficients are estimated in the linear least squares sense. Estimated coefficients are examined and grouped with respect to different kinds of auroral storms. Examples of modeling methodology are provided using four different kinds of intense storms and substorms, namely, Positive Arctic, Positive Antarctic, Negative Arctic and Negative Antarctic that occurred between 1999 and 2013. The estimated coefficients for storm periods are compared with those of non-storm periods. It is observed that the positive correlation between the increase of AE and GEC can be a promising precursor of space weather variability.

Butterfly pitch angle distribution of relativistic electrons in the outer radiation belt: Evidence of nonadiabatic scattering

AbstractIn this paper we investigate the scattering of relativistic electrons in the nightside outer radiation belt (around the geostationary orbit). We consider the particular case of low geomagnetic activity (|Dst|<20 nT), quiet conditions in the solar wind, and absence of whistler wave emissions. For such conditions we find several events of Van Allen probe observations of butterfly pitch angle distributions of relativistic electrons (energies about 1–3 MeV). Many previous publications have described such pitch angle distributions over a wide energy range as due to the combined effect of outward radial diffusion and magnetopause shadowing. In this paper we discuss another mechanism that produces butterfly distributions over a limited range of electron energies. We suggest that such distributions can be shaped due to relativistic electron scattering in the equatorial plane of magnetic field lines that are locally deformed by currents of hot ions injected into the inner magnetosphere. Analytical estimates, test particle simulations, and observations of the AE index support this scenario. We conclude that even in the rather quiet magnetosphere, small scale (magnetic local time (MLT)‐localized) injection of hot ions from the magnetotail can likely influence the relativistic electron scattering. Thus, observations of butterfly pitch angle distributions can serve as an indicator of magnetic field deformations in the nightside inner magnetosphere. We briefly discuss possible theoretical approaches and problems for modeling such nonadiabatic electron scattering.

Characteristics of PMSE associated with the geomagnetic disturbance driven by corotating interaction region and high‐speed solar wind streams in the declining solar cycle 23

AbstractWe report interannual variations of the correlation between the reflectivity of polar mesospheric summer echoes (PMSEs) and solar wind parameters (speed and dynamic pressure), and AE index as a proxy of geomagnetic disturbances, and cosmic noise absorption (CNA) in the declining phase (2001–2008) of solar cycle 23. PMSEs are observed by 52 MHz VHF radar measurements at Esrange (67.8°N, 20.4°E), Sweden. In approaching the solar minimum years, high‐speed solar wind streams emanate from frequently emerging coronal holes, leading to 7, 9, and 13.5 day periodicities in their arrival at Earth. Periodicities of 7 and/or 9 days are found in PMSE reflectivity in 2005–2006 and 2008. Periodicity‐resolved correlations at 7 and 9 days of both D region ionization observed by cosmic noise absorption (CNA) and PMSE with solar wind speed and AE index vary from year to year but generally increase as solar minimum is approached. PMSEs have a higher periodicity‐resolved correlation with AE index than the solar wind speed. In addition, cross correlation of PMSE reflectivity with AE index is mostly higher than with CNA in solar minimum years (2005–2008). This can signify that high‐speed solar wind stream‐induced high‐energy particles possibly have strong influence on CNA, but not as much as on PMSE, especially for the years of significant periodicities occurring.

A prediction model for the global distribution of whistler chorus wave amplitude developed separately for two latitudinal zones

AbstractWhistler mode chorus waves are considered to play a central role in accelerating and scattering electrons in the outer radiation belt. While in situ measurements are usually limited to the trajectories of a small number of satellites, rigorous theoretical modeling requires a global distribution of chorus wave characteristics. In the present work, by using a large database of chorus wave observations made on the Time History of Events and Macroscale Interactions during Substorms satellites for about 5 years, we develop prediction models for a global distribution of chorus amplitudes. The development is based on two main components: (a) the temporal dependence of average chorus amplitudes determined by correlating with the preceding solar wind and geomagnetic conditions as represented by the interplanetary magnetic field (IMF) Bz and AE index and (b) the determination of spatial distribution pattern of chorus amplitudes, specifically, the profiles in L in all 2 h magnetic local time zones, which are categorized by activity levels of either the IMF Bz or AE index. Two separate models are developed: one based only on the IMF Bz and the other based only on AE. Both models predict chorus amplitudes for two different latitudinal zones separately: |magnetic latitude (MLAT)| < 10°, and |MLAT| = 10°–25°. The model performance is measured by the coefficient of determination R2 and the rank‐order correlation coefficient (ROCC) between the observations and model prediction results. When tested for a new data interval of ~1.5 years, the AE‐based model works slightly better than the IMF Bz‐based model: for the AE‐based model, the mean R2 and ROCC values are ~0.46 and ~0.78 for |MLAT| < 10°, respectively, and ~0.4 and ~0.74 for |MLAT| = 10°–25°, respectively; for the IMF Bz‐based model, the mean R2 and ROCC values are ~0.39 and ~0.74 for |MLAT| < 10°, respectively, and ~0.33 and ~0.70 for |MLAT| = 10°–25°, respectively. We provide all of the model information in the text and supporting information so that the developed chorus models can be used for the existing outer radiation belt electron models.

A Statistical Analysis of Solar Wind Parameters and Geomagnetic Indices during the Solar Cycle 23

AbstractInterplanetary Coronal mass ejections (CMEs) and corotating interaction regions (CIRs) are two types of most geoeffective solar wind transients and also the main drivers of geomagnetic storms. In this study, by using a superposed epoch analysis, we have compared the statistical features of solar wind parameters for the two types of solar disturbances during the 23rd solar cycle and the resulting changes of geomagnetic indices (Dst, AE, and Kp). Meanwhile, the statistics of solar wind parameters and geomagnetic indices corresponding to magnetic storms with different intensities are also investigated. In general, it is found that the slopes of linearly fitted Nsw/Pdyn curves (where Nsw is the solar wind proton density and Pdyn is the solar wind dynamic pressure) with respect to epoch time remains positive for CME events but negative for CIR events, which can act as a feasible means to distinguish CME and CIR events. On average, compared to CIR events, CME events have larger magnitudes of southward IMF Bz, solar wind dynamic pressure, AE and Kp indices but smaller Dstmin. In principle, CMEs bear higher possibility to drive extremely intense (i.e., super) geomagnetic storms. The overall variations of Dst tend to be similar to some extent for different levels of geomagnetic storms, however, Dst decreases faster for stronger storms.

North‐south asymmetry of the high‐latitude thermospheric density: IMF BY effect

AbstractPrevious studies have established that the y component of the interplanetary magnetic field (IMF By) plays a role in the north‐south asymmetry of the high‐latitude plasma convection and wind. The effect of the positive/negative IMF By in the Northern Hemisphere resembles the effect that the negative/positive IMF By would have in the Southern Hemisphere. In this study, we demonstrate that the IMF By effect can also contribute to the hemispheric asymmetry of the thermospheric density. We use high‐accuracy air drag measurements from the CHAllenging Minisatellite Payload (CHAMP) satellite and SuperMAG AE index during the period 2001–2006 to examine the response of the high‐latitude thermospheric density to geomagnetic activity. Our statistical analysis reveals that the density response at 400 km is greater in the Southern Hemisphere under positive IMF By conditions, and greater in the Northern Hemisphere under negative IMF By conditions. The results suggest that the IMF By effect needs to be taken into account in upper atmospheric modeling for an accurate description of high‐latitude densities during periods of enhanced geomagnetic activity.

Solar wind turbulence as a driver of geomagnetic activity

We carried out simultaneous analyses of interplanetary and geomagnetic datasets for the period of (solar Maunder) least (2009) and maximum (2002) solar activity to determine the nature of solar wind turbulence on geomagnetic activity using AE, ASY-D, and ASY-H indices. We determined the role played by Alfvénic fluctuations in the solar wind so as to find out the nature of the turbulence. Our analyses showed that solar wind turbulence play a role in geomagnetic processes at high latitudes during periods of low and high solar activity but does not have any effect at mid-low latitudes.

TWINS stereoscopic imaging of multiple peaks in the ring current

AbstractGlobal, ion equatorial flux distributions and energy spectra are presented from stereoscopic Two Wide‐Angle Imaging Neutral‐Atom Spectrometers (TWINS) 1 and TWINS 2 energetic neutral atom (ENA) images for two time periods, 29 May 2010, 1330–1430 UT and 26 May 2011, 1645–1715 UT. The first is just after the main phase of a weak (minimum SYM/H ≈ −70 to −80 nT) corotating interaction region‐driven geomagnetic storm. The second is during a relatively quiet period. The global ion distributions show multiple spatial peaks that are coincident with peaks in the AE index. The energy spectra have a primary maximum in the 15–20 keV range. Below the energy maximum, the flux is Maxwellian. Above the main maximum, the flux is either significantly below that of a Maxwellian or has a second component with a maximum in the 40–50 keV range. For the 29 May 2010, 1330–1430 UT time period, the flux from the TWINS stereoscopic images is compared to the results from TWINS 1 and TWINS 2 alone illustrating the advantage of stereoscopic viewing. The flux deconvolved from the TWINS images also shows spatial and temporal correlations with Time History of Events and Macroscale Interactions during Substorms (THEMIS) in situ measurements. Magnetic field dipolarizations observed by GOES support the existence of a peak in the ion flux in the midnight/dawn sector. In summary, increased spatial resolution from TWINS stereoscopic ENA images is demonstrated. Multiple peaks in the ion flux of trapped particles in the ring current are observed. THEMIS electrostatic analyzer in situ ion flux measurements and GOES geosynchronous magnetic field measurements are consistent with the spatial and temporal structure obtained.

Auroral electrojets during severely disturbed geomagnetic condition on 24 August 2005

Very intense and highly dynamic eastward and westward currents flowing in the auroral ionosphere are traditionally monitored by the auroral electrojet indices – AUandAL, respectively. In this study we show that on occasions of intense magnetic activity, entire auroral oval could be dominated by the westward flowing currents, which lead to depression not only in AL index but also in supposedly positive AU index. During negative AU intervals, there could be up to ∼20% underestimation of the total maximum intensity of the auroral electrojet represented by AEindex(defined asAU-AL). A detailed investigation of a well-studied extremely intense event of 24 August 2005 has been carried out. Global prevalence of the westward auroral electrojet was clearly observed at the auroral latitudes during the unusually intense substorm (AL∼-4000nT) on the day. Moreover, along the noon meridian westward electrojet appeared in the auroral region whereas eastward electrojet shifted towards lower latitudes. This paper emphasizes that intense substorms are represented better by AL index than AE index.

Investigation of sudden electron density depletions observed in the dusk sector by the Poker Flat, Alaska incoherent scatter radar in summer

AbstractThis paper investigates unusually deep and sudden electron density depletions (troughs) observed in the Poker Flat (Alaska) Incoherent Scatter Radar data in middle summer of 2007 and 2008. The troughs were observed in the premidnight sector during periods of weak magnetic and solar activity. The density recovered to normal levels around midnight. At the time when the electron density was undergoing its steep decrease, there was usually a surge of the order of 100 to 400 K in the ion temperature that lasted less than 1 h. The Ti surges were usually related to similar surges in the AE index, indicating that the high‐latitude convection pattern was expanding and intensifying at the time of the steep electron density drop. The convection patterns from the Super Dual Auroral Radar Network also indicate that the density troughs were associated with the expansion of the convection pattern to Poker Flat. The sudden decreases in the electron density are difficult to explain in summer because the high‐latitude region remains sunlit for most of the day. This paper suggests that the summer density troughs result from lower latitude plasma that had initially been corotating in darkness for several hours post sunset and brought back toward the sunlit side as the convection pattern expanded. The magnetic declination of ~22° east at 300 km at Poker Flat greatly facilitates the contrast between the plasma convecting from lower latitudes and the plasma that follows the high‐latitude convection pattern.