Low Solar Activity Levels Research Articles

An Update of the NeQuick-Corr Topside Ionosphere Modeling Based on New Datasets

A new analytical formula for H0, one of the three parameters (H0, g, and r) on which the NeQuick model is based to describe the altitude profile of the electron density above the F2-layer peak height hmF2, has recently been proposed. This new analytical representation of H0, called H0,corr, relies on numerical grids based on two different types of datasets. On one side, electron density observations by the Swarm satellites over Europe from December 2013 to September 2018, and on the other side, IRI UP (International Reference Ionosphere UPdate) maps over Europe of the critical frequency of the ordinary mode of propagation associated with the F2 layer, foF2, and hmF2, at 15 min cadence for the same period. The new NeQuick topside representation based on H0,corr, hereafter referred to as NeQuick-corr, improved the original NeQuick topside representation. This work updates the numerical grids of H0,corr by extending the underlying Swarm and IRI UP datasets until December 2021, thus allowing coverage of low solar activity levels, as well. Moreover, concerning Swarm, besides the original dataset, the calibrated one is considered, and corresponding grids of H0,corr calculated. At the same time, the role of g is investigated, by considering values different from the reference one, equal to 0.125, currently adopted. To understand what are the best H0,corr grids to be considered for the NeQuick-corr topside representation, vertical total electron content data for low, middle, and high latitudes, recorded from five low-Earth-orbit satellite missions (COSMIC/FORMOSAT-3, GRACE, METOP, TerraSAR-X, and Swarm) have been analyzed. The updated H0,corr grids based on the original Swarm dataset with a value for g = 0.15, and the updated H0,corr grids based on the calibrated Swarm dataset with a value for g = 0.14, are those for which the best results are obtained. The results show that the performance of the different NeQuick-corr models is reliable also for low latitudes, even though these are outside the spatial domain for which the H0,corr grids were obtained, and are dependent on solar activity.

Unveiling the Core Patterns of High-Latitude Electron Density Distribution at Swarm Altitude

The ionosphere has distinctive characteristics under different solar and geomagnetic conditions, as well as throughout the seasons, and has a direct impact on our technological life in terms of radio communication and satellite navigation systems. In the pursuit of developing highly accurate ionospheric models and/or improving existing ones, understanding the various physical mechanisms that influence electron density dynamics is critical. In this study, we apply the Multivariate Empirical Mode Decomposition (MEMD) method to the electron density distribution in the mid-to-high latitude (above 50° magnetic latitude) regions in order to identify the dominant scales at which these mechanisms operate. The data were collected by the Swarm mission in the Northern Hemisphere. MEMD allows us to separate the main intrinsic modes and assess their relative contributions to the original one, thereby identifying the most important modes and the spatial scales at which they exert influence. Our study spanned the period from 1 January 2016 to 31 December 2021, which was characterized by low solar activity levels. This choice allowed for a more focused investigation of other variables influencing electron density distribution under similar solar activity conditions. We specifically examined the variations of the resulting modes in relation to different seasons and geomagnetic activity conditions, providing valuable insights into the complex behavior of the ionosphere in response to various external factors.

Parametric study on the flight envelope of a radio-frequency ion thruster based atmosphere-breathing electric propulsion system

The atmosphere-breathing electric propulsion (ABEP) system utilizes atmospheric species as propellants to generate thrust for drag compensation of a satellite in very-low-Earth-orbit (VLEO). A parametric approach is used to assess the impact of different parameters on the flight envelope of a sample VLEO satellite with an ABEP system based on a radio-frequency ion thruster (RIT). The considered parameters include capture efficiency, maximum input power, solar activity, and atomic oxygen recombination factor. The NRLMSIS 2.0 atmosphere model is used to determine the flow conditions at VLEO, at a target altitude of 150–300 km with high, moderate, and low solar activity levels. DSMC method is employed to calculate the drag of the sample satellite with the ABEP system. The 0-D model of the RIT discharge chamber is used to predict the thrust of the RIT-based ABEP system. The flight envelope of a sample RIT-based ABEP system shows that drag can be compensated at altitudes between 196 km and 248 km. Increasing capture efficiency and maximum input power expands the feasible range for drag compensation to higher and lower altitudes, respectively. Also, the flight envelope shifts to higher altitudes with increasing solar activity levels. However, the atomic oxygen recombination factor of the intake device has minimal effects on the flight envelope.

Investigation of the planar midlatitude ionospheric trend under different levels of solar activity

A better understanding of the ionosphere through accurate mathematical models is no doubt a crucial element. This study focuses on the challenging problem of building a model representing the complex structure of the midlatitude ionosphere. Previous studies have shown that a regional planar model is suitable in representing the total electron content (TEC) trend in the midlatitude ionosphere in both hemispheres. In this study, the planar trend model for 12 non-overlapping northern hemisphere regions in three groups of geographically near 4 regions is further investigated under different levels of solar activity; low, moderate and high. To that end, the coefficients of the model are estimated in the least squares sense using total electron content values from global ionospheric maps (GIMs) for the years 2009, 2012 and 2014. Subsequently, these coefficients are used to reconstruct estimated TEC maps which are then compared with actual GIM-TEC by investigating their difference in normalized L2 norm squared sense. The regional planar trend model provides a particularly successful representation in the years 2012 and 2014 for which the solar activity level is the dominant factor determining the TEC trend. Under low solar activity conditions of 2009, other factors such as ocean currents, temperature variations and meteorological phenomena are suspected to have a considerable effect in some regions depending on their geographic location and on seasonal trends in those regions. As an example, studies show that under the influence of the Pacific Decadal Oscillation (PDO) and Siberian High (SH), a significant cooling trend between 2004 and 2018 in autumn is observed in Eurasia, which, in conjunction with the low solar activity levels, may be related to the deviations from the actual GIM-TEC in 2009 in these regions. As solar radiation increases, however, such bottom-side forcings are masked in 2012 and 2014 and these deviations are no longer observed.

First Observations of Anomalous Cosmic Rays in to 36 Solar Radii

Abstract NASA’s Parker Solar Probe mission continues to travel closer to the Sun than any prior human-made object, with an expected closest approach of <10 solar radii (<0.046 au) by 2024. On board, the Integrated Science Investigation of the Sun instrument suite makes unprecedented in situ measurements of energetic particles in the near-Sun environment. The current low level of solar activity offers a prime opportunity to measure cosmic rays closer to the Sun than ever before. We present the first observations of anomalous cosmic rays in to 36 solar radii (0.166 au), focusing specifically on helium. Our results indicate a strong radial intensity gradient of ∼25 ± 5%/au over energies of ∼4 to ∼45 MeV/nuc. These values are larger than prior observations, further out in the heliosphere, and come at a unique time in our understanding and modeling of particle transport and acceleration, particularly as both Voyagers have crossed the heliopause and IBEX has accumulated a full solar cycle of observations. Thus, continued measurements of cosmic rays by Parker Solar Probe will play a critical role in linking past observations with our present knowledge and significantly advancing our understanding of cosmic ray transport in the heliosphere.

Strong evidence of low levels of solar activity during the Maunder Minimum

ABSTRACT The Maunder Minimum (MM) was a period of prolonged solar activity minimum between 1645 and 1715. Several works have identified a significant number of problematic spotless days in the MM included in existing data bases. We have found a list of exact spotless (in the second half of 1709) and spot days (January and August 1709) provided by Johann Heinrich Müller. We computed the most probable value and upper/lower limits of the active day fraction (ADF) from Müller's data using the hypergeometrical probability distribution. Our sample is not strictly random because Müller recorded observations in consecutive days when he observed sunspots. Therefore, our result represents an upper threshold of solar activity for 1709. We compared this result with annual values of the ADF calculated for the Dalton Minimum and the most recent solar cycles. We concluded that, although 1709 is one of the most active years in the MM, it was less active than most years both in the Dalton Minimum and in the most recent solar cycles. Therefore, the solar activity level estimated in this work for 1709 represents robust evidence of low solar activity levels in the MM.

Assessment of IRI-2016 hmF2 model options with digisonde, COSMIC and ISR observations for low and high solar flux conditions

The attempt of this work is to evaluate the three options (BSE-1979, AMTB-2013, SHU-2015) embedded in IRI-2016 for the F2 layer peak height (hmF2) estimation under low and high solar flux levels. We compare the model hmF2 with digisonde observations from Qaanaaq, Milllstone Hill, Ramey, Jicamarca and Port Stanley as well as Constellation Observing System for Meteorology, Ionosphere and Climate (COSMIC) ionospheric radio occultation data over the five stations spanning from northern hemisphere to southern hemisphere. In the case of Millstone Hill, we also use incoherent scatter radar (ISR) measurements. For the comparison with digisonde data, SHU-2015 is the most accurate option for reproducing hmF2 at low and middle latitude stations (Milllstone Hill, Ramey, Jicamarca) with the smallest Root-Mean-Square-Error and the highest correlation with observations. At high latitude (Qaanaaq and Port Stanley), BSE-1979 becomes the best option. This is true for both low and high solar flux conditions. Similar results are obtained for the comparison between ISR and IRI-2016 hmF2 model options over Millstone Hill, where SHU-2015 option has the smallest deviations from the observation data in both low and high solar activity conditions. As for the comparison with COSMIC data, SHU-2015 edges out the other two options under low solar activity level with better performance at three stations. There is no optimal option under high solar activity level, which is to our surprise, cause SHU-2015 was developed based on a large amount of radio occultation data. At Jicamarca, all options show dramatic deviations from COSMIC observations, showing even negative correlations. We propose that the performance of the three options do not vary much with solar condition, while they differ a lot with location, for example, latitude.

Вплив глобальної сейсмічної активності на процеси в атмосфері й іоносфері

In recent decades, ideas about earthquakes (EQ) have been formed as a final stage of a planetary continuous self-organizing tectonic process with periods of accumulation and relaxation of tectonic stresses. However, the scientific literature still presents studies of the response of atmospheric and ionospheric processes to individual strong EQs. In this paper, for the first time, the relationship between processes in the lithosphere, troposphere, and ionosphere is considered, taking into account new ideas about the seismic process as a global phenomenon and on the background of processes caused by space weather. Both planetary data (EQ, total electron content (TEC) of the ionosphere) and data (atmospheric pressure, critical frequency of the F2 layer of the ionosphere) of widely spaced observation points in the western and eastern hemispheres were used. To increase the reliability of statistical results, 4 independent databases of daily data for 2007–2015 were used. Stable effects of global seismic activity (GSA) in the considered parameters are established. Thus, the critical frequency of the F2 region with a sharp increase in the GSA increases by 0.4–0.5 MHz. This effect is quite stable and manifests itself almost simultaneously at ionospheric stations of the eastern and western hemispheres, as well as in planetary TEC values. At the same time, in the ionospheric variations, as before, the influence of both the troposphere (especially at a low level of solar activity) and space weather is traced, the characteristics of which in 75 % of cases also show an association with GSA. Therefore, space weather often but not always can act as a trigger on the EQs. In general, in the western hemisphere, the minimum atmospheric pressure occurs earlier than in the eastern, which leads to a noticeable increase in the pressure difference between the hemispheres by 10 mm. Hg., that indicates the relationship between global seismicity and global atmospheric circulation. The established GSA effects, as a rule, have the character of not a local short-term burst, but a jump followed by a gradual decrease (increase) of the index until the next active period (saw-toothed curve), i.e., the influence of the lithosphere on the overlying layers is continuous and is cyclical in nature, probably due to the cyclical nature of tectonic processes. Most likely, several different couplings between geospheres are realized at the same time, partially synchronized by changes in space weather, which requires new physical mechanisms to explain them.

Effects of solar activity on ionospheric current system in the Southeast Asia region

The equatorial ionospheric current system is composed of equatorial electrojet (EEJ) and solar quiet (Sq) that flow in the eastward direction. That being mentioned, this study looked into the changes in ionospheric currents stemming from geomagnetic storm disturbances at different levels of solar activity during solar cycle 24. This analysis was performed by employing an EEJ index known as EUEL, which is calculated from northward geomagnetic component data measured using ground-based magnetometer at five different latitudes across the Southeast Asia region. Based on the outcomes, the impacts of geomagnetic storm on EEJ currents were observed at 1200 LT and 1100 LT, respectively, during high and low solar activity levels. Time delay was associated with changes in EEJ peak time at varied activity levels. This case study reveals that EEJ value at peak time of high solar activity decreased significantly during the main phase of geomagnetic storm.

Influence of Solar Activity on Penetration of Traveling Planetary‐Scale Waves From the Troposphere Into the Thermosphere

AbstractIn this study, numerical simulations of planetary‐scale waves (PSWs), generated in the troposphere, were performed for altitudes from the Earth's surface up to 300 km. The influence of thermospheric effects of solar activity (SA) on the amplitudes and phases of westward traveling PSWs with zonal wave numbers 1 and 2 and periods 4–16 days propagating from the troposphere was simulated. Such simulations for a large number of PSW modes in the thermosphere were made for the first time. The effects of SA changes at altitudes above 100 km were involved in the general circulation model MUAM. The ionospheric conductivities for minimum and maximum SA levels were included into the MUAM. The simulation results were averaged over two ensembles of model runs with different PSW phases for conditions corresponding to the high and low SA levels for January–February. PSW atmospheric refractivity index and Eliassen‐Palm flux were calculated. They correspond to simulated changes in PSW amplitudes. Changes in the zonal velocity and temperature caused by the SA variations can modify spatial distributions of the westward traveling PSWs. Wave amplitudes significantly (up to 100%) decrease at the thermospheric heights under high SA, which is accompanied by decreasing vertical component of the Eliassen‐Palm flux. The 7‐, 10‐, and 16‐day PSWs could have larger partial reflection and worse propagation conditions than the 4‐ and 5‐day waves in the Southern Hemisphere under the high SA. At altitudes below 100 km, minor differences in zonal velocity and PSW amplitudes between high and low SA are found.

The expected fluxes observed by STIX during low solar activity

The Spectrometer Telescope for Imaging X-rays (STIX) is one of the instruments installed onboard Solar Orbiter mission which will be launched in February 2020. After 1.5 years of cruise phase it will start to gather scientific data from the orbit with perihelion distance about 0.28 au. It means that STIX will operate also during the next solar minimum. In the paper we estimate flux measured by the instrument during periods of low solar activity. For this purpose we used solar observations which were recorded by the Solar Photometer in X-rays (SphinX) during the last minimum of solar activity. The estimation was obtained for instruments overlapping energy range from 4 to 15 keV. Presented results indicate that STIX instrument will provide efficient imaging the solar emission even during low level of solar activity (B1 GOES class).

Statistical Behavior of the Longitudinal Variations of the Evening Topside Mid‐Latitude Trough Position in both Northern and Southern Hemispheres

AbstractBased on the nighttime ionosphere ion density data from the Defense Meteorological Satellite Program (DMSP) satellites in 1996–2016, we statistically analyzed the longitudinal variation of the mid‐latitude trough position in the Northern and Southern hemispheres. The results show significant difference in the longitudinal variation of the trough position between the Northern and Southern hemispheres. In the Northern hemisphere, the trough position in the two longitudinal sectors of 60°W and 120°E is always located at higher latitudes than that in the other longitudinal sectors under most of time and solar and geomagnetic conditions. The longitudinal variation of trough position in the Northern hemisphere is mainly affected by season, solar activity level, and geomagnetic activity level. In the Southern hemisphere, the trough position in the western hemisphere is located at lower latitude than that in the eastern hemisphere in winter and at low solar activity level in equinox. At medium and high solar activity levels in equinox, the southern trough structure in the western hemisphere can't be identified, which also occurs in summer. The change in geomagnetic activity would cause the same movement of trough position in the Southern hemisphere in all longitude sectors at low solar activity level. Thus, the longitudinal variation of trough position in the Southern hemisphere is mainly affected by season and solar activity level. The longitudinal variation of the trough position may be associated with the longitudinal variation of the ion density at the equatorward/poleward side of the trough in the Northern and Southern hemispheres.

Solar rotational cycle in lightning activity in Japan during the 18–19th centuries

Abstract. Thunderstorm and cloud activities sometimes show a 27-day period, and this has long been studied to uncover a possible important link to solar rotation. Because the 27-day variations in the solar forcing parameters such as solar ultraviolet and galactic cosmic rays become more prominent when the solar activity is high, it is expected that the signal of the 27-day period in meteorological phenomena may wax and wane according to the changes in the solar activity level. In this study, we examine in detail the intensity variations in the signal of the 27-day solar rotational period in thunder and lightning activity from the 18th to the 19th centuries based on 150-year-long records found in old diaries kept in Japan and discuss their relation with the solar activity levels. Such long records enable us to examine the signals of solar rotation at both high and low solar activity levels. We found that the signal of the solar rotational period in the thunder and lightning activity increases as the solar activity increases. In this study, we also discuss the possibility of the impact of the long-term climatological conditions on the signals of the 27-day period in thunder/lightning activities. Keywords. Meteorology and atmospheric dynamics (lightning)

Could a Hexagonal Sunspot Have Been Observed During the Maunder Minimum?

The Maunder Minimum was the period between 1645 and 1715 whose main characteristic was abnormally low and prolonged solar activity. However, some authors have doubted this low level of solar activity during that period by questioning the accuracy and objectivity of the observers. This work presents a particular case of a sunspot observed during the Maunder Minimum with an unusual shape of its umbra and penumbra: a hexagon. This sunspot was observed by Cassini in November 1676, just at the core of the Maunder Minimum. This historical observation is compared with a twin case that occurred recently in May 2016. The conclusion reached is that Cassini's record is another example of the good quality observations made during the Maunder Minimum, showing the meticulousness of the astronomers of that epoch. This sunspot observation made by Cassini does not support the conclusions of Zolotova and Ponyavin (Astrophys. J. 800, 42, 2015) that professional astronomers in the 17th century only registered round sunspots. Finally, a discussion is given of the importance of this kind of unusual sunspot record for a better assessment of the true level of solar activity in the Maunder Minimum.

The Impact of Ionospheric and Geomagnetic Changes on Mortality from Diseases of the Circulatory System

We investigate the impact of solar activity changes on mortality from cardiovascular causes of death in the period 1994-2011 in the Czech Republic. This period coincides with the time of solar cycle no. 23 and the surrounding minima when there was an unusually low level of solar activity. We use long-period daily time series of numbers of deaths by cause, solar activity indices (the relative sunspot number, and the intensity of solar radio flux), geomagnetic indices (Kp-the planetary index that indicates the fluctuation rate of horizontal components of the geomagnetic field, the Auroral Electrojet, and the disturbance storm time), and physical parameters describing the ionospheric effects (the critical frequency of the ionospheric F2 layer and the content of free electrons in the ionosphere). The results of the analysis confirm the hypothesis that there is no direct correlation between the geomagnetic solar index, Kp, and the number of deaths from acute myocardial infarction (code I21) or brain stroke (code I64) during the maxima of the solar cycle. On the other hand, the ionospheric parameters explain a greater part of the variability in the number of deaths for acute myocardial infarction or brain stroke than the model with solar parameters. The analysis shows that, because the values are geographically specific, the ionospheric parameters may describe the variability in the number of deaths from cardiovascular causes better than the solar indices. The cardiovascular diseases thus respond to the changes in the solar activity and to abnormal solar events indirectly through a concentration of electrical charges in the earth's environment.

LONG-TERM TREND OF SUNSPOT NUMBERS

ABSTRACT Using the Hilbert–Huang Transform method, we investigate the long-term trend of yearly mean total sunspot numbers in the time interval of 1700–2015, which come from the World Data Center—the sunspot Index and long-term solar observations. The main findings of this study are summarized below. (1) From the adaptive trend, which is extracted from the yearly mean total sunspot numbers, we can find that the value gradually increases during the time period 1700–1975, then decreases gradually from 1975 to 2015. (2) The Centennial Gleissberg Cycle is extracted from the yearly mean total sunspot numbers and confirms that a new grand minimum is in progress; the Dalton Minimum, the Gleissberg Minimum, and low level of solar activity during solar cycle 24 (the part of the new grand minimum) all can be understood as minima of the Centennial Gleissberg Cycle. (3) Based on the adaptive (overall) trend, and the 100-year and longer timescale trend of yearly mean total sunspot numbers, we can infer that the level of solar activity during the new grand minimum may be close to that during the Gleissberg Minimum, slightly higher than that during the Dalton Minimum, and significantly higher than that during the Maunder Minimum. Our results do not support the suggestion that a new grand minimum, somewhat resembling the Maunder Minimum, is in progress.

Sudden stratospheric warmings: statistical characteristics and influence on NO2 and O3 total contents

Statistical characteristics of major and minor sudden stratospheric warmings (SSWs) in the Northern Hemisphere (NH) for 1958–2015 are analyzed using data of NCEP-NCAR, ERA 40, and ERA-Interim reanalyses. Dependencies of the number of major SSWs with the displacement of the circumpolar stratospheric vortex and the number of minor SSWs on the phase of the quasi-biennial oscillation (QBO) of the equatorial stratospheric wind and on the level of solar activity (SA) in the 11-year solar cycle have been revealed. Major SSWs accompanied by a displacement of the polar vortex occur more often at a high level of SA and at the easterly phase of the QBO in the 50–40 hPa layer, while minor SSWs occur more often at a low SA level and at the westerly phase of the QBO. An analysis of spatiotemporal dynamics of the stratospheric polar vortex at major SSWs is performed. The most probable directions of vortex displacement caused by SSWs have been revealed. Influences of the major SSWs on the total contents of NO2 and ozone, as well as on stratosphere temperature, are analyzed.

Dusk-to-nighttime enhancement of mid-latitude <i>Nm</i>F2 in local summer: inter-hemispheric asymmetry and solar activity dependence

Abstract. In this paper ionosonde observations in the East Asia–Australia sector were collected to investigate dusk-to-nighttime enhancement of mid-latitude summer NmF2 (maximum electron density of the F2 layer) within the framework of NmF2 diurnal variation. NmF2 were normalized to two solar activity levels to investigate the dependence of the dusk-to-nighttime enhancement on solar activity. The dusk-to-nighttime enhancement of NmF2 is more evident at Northern Hemisphere stations than at Southern Hemisphere stations, with a remarkable latitudinal dependence. The dusk-to-nighttime enhancement shows both increasing and declining trends with solar activity increasing, which is somewhat different from previous conclusions. The difference in the dusk-to-nighttime enhancement between Southern Hemisphere and Northern Hemisphere stations is possibly related to the offset of the geomagnetic axis from the geographic axis. hmF2 (peak height of the F2 layer) diurnal variations show that daytime hmF2 begins to increase much earlier at low solar activity level than at high solar activity level at northern Akita and Wakkanai stations where the dusk-to-nighttime enhancement is more prominent at low solar activity level than at high solar activity level. That implies neutral wind phase is possibly also important for nighttime enhancement.

STATISTICAL CHARACTERISTICS OF ELEMENTAL ABUNDANCE RATIOS: OBSERVATIONS FROM THEACESPACECRAFT

We statistically analyze the elemental galactic cosmic ray (GCR) composition measurements of elements 5 ≤ Z ≤ 28 within the energy range 30–500 MeV/nucleon from the CRIS instrument on board the ACE spacecraft in orbit about the L1 Lagrange point during the period from 1997 to 2014. Similarly to the last unusual solar minimum, the elevated elemental intensities of all heavy nuclei during the current weak solar maximum in 2014 are ~40% higher than that of the previous solar maximum in 2002, which has been attributed to the weak modulation associated with low solar activity levels during the ongoing weakest solar maximum since the dawn of space age. In addition, the abundance ratios of heavy nuclei with respect to elemental oxygen are generally independent of kinetic energy per nucleon in the energy region 60–200 MeV/nuc, in good agreement with previous experiments. Furthermore, the abundance ratios of most relatively abundant species, except carbon, exhibit considerable solar-cycle variation, which are obviously positively correlated with the sunspot numbers with about one-year time lag. We also find that the percentage variation of abundance ratios for most elements are approximately identical. These preliminary results provide valuable insights into the characteristics of elemental heavy nuclei composition and place new and significant constraints on future GCR heavy nuclei propagation and modulation models.

Identifying equatorial ionospheric irregularities using in situ ion drifts

Abstract. Previous climatological investigations of ionospheric irregularity occurrence in the equatorial ionosphere have utilized in situ measurements of plasma density to identify the presence of an irregularity. Here we use the Morlet wavelet and C/NOFS to isolate perturbations in meridional ion drifts and generate irregularity occurrence maps as a function of local time, longitude, season, and solar activity. For the low solar activity levels in 2008, the distributions identified by velocity perturbations follow normalized density perturbation (ΔN/N) maps with large occurrences after midnight into dawn over all longitudes. The velocity and normalized density occurrence maps contract in both local time and longitude with increasing solar activity. By 2011 irregularities are confined to particular longitudes expected by alignment and a few hours of local time after sunset. The variation in the occurrence of the late night irregularities with solar activity is consistent with the presence of gravity wave seeding.