Equatorial Ionospheric Irregularities Research Articles

Equinoctial asymmetry of a low-latitude ionosphere-thermosphere system and equatorial irregularities: evidence for meridional wind control

Abstract. Nocturnal ionospheric height variations were analyzed along the meridian of 100° E by using ionosonde data. Two ionosondes were installed near the magnetic conjugate points at low latitudes, and the third station was situated near the magnetic equator. Ionospheric virtual heights were scaled every 15 min and vertical E×B drift velocities were inferred from the equatorial station. By incorporating the inferred equatorial vertical drift velocity, ionospheric bottom heights with the absence of wind were modeled for the two low-latitude conjugate stations, and the deviation in heights from the model outputs was used to infer the transequatorial meridional thermospheric winds. The results obtained for the September and March equinoxes of years 2004 and 2005, respectively, were compared, and a significant difference in the meridional wind was found. An oscillation with a period of approximately 7 h of the meridional wind existed in both the equinoxes, but its amplitude was larger in September as compared to that in March. When the equatorial height reached the maximum level due to the evening enhancement of the zonal electric field, the transequatorial meridional wind velocity reached approximately 10 and 40 m/s for the March and September equinoxes, respectively. This asymmetry of the ionosphere-thermosphere system was found to be associated with the previously reported equinoctial asymmetry of equatorial ionospheric irregularities; the probability for equatorial irregularities to occur is higher in March as compared to that in September at the Indian to Western Pacific longitudes. Numerical simulations of plasma bubble developments were conducted by incorporating the transequatorial neutral wind effect, and the results showed that the growth time (e-folding time) of the bubble was halved when the wind velocity changed from 10 to 40 m/s.

Cause of different local time distribution in the postsunset equatorial ionospheric irregularity occurrences between June and December solstices

Global averaged postsunset equatorial ionospheric density irregularity occurrences observed by ROCSAT during the moderate to high solar activity years of 1999 to 2004 indicate a different local time distribution between June and December solstices. The irregularity occurrences during the December solstice show a faster increase rate to peak at 2100–2200 local time, while the irregularity occurrences during the June solstice have a slower increase rate and peak one hour later in local time than that in the December solstice. The cause of such different local time distributions is attributed to a large contrast in the time of zonal drift reversal and the magnitude of postsunset vertical drift observed by ROCSAT at longitudes of large magnetic declination in the two solstices. That is, a delay in the zonal drift reversal in association with a smaller postsunset vertical drift observed at longitudes of positive magnetic declination has greatly inhibited the irregularity occurrences during the June solstice in contrast to an earlier zonal drift reversal together with a large vertical drift occurring at longitudes of negative magnetic declination to accelerate the irregularity occurrences during the December solstice. We think that the different geomagnetic field strengths that existed between the longitudes of positive and negative magnetic declinations have played a crucial role in determining the different local time distributions of irregularity occurrences for the two solstices.

Day‐to‐day variability in the development of plasma bubbles associated with geomagnetic disturbances

The development of equatorial ionospheric irregularities into large‐scale ionospheric plasma bubbles continues to be an active area for scientific investigations. In this paper, we present simultaneous OI 630.0‐nm emission all‐sky imaging observations carried out at the Astrophysics National Laboratory (LNA), Brazopolis (22.5°S, 45.6°W, altitude 1860 m) and ionospheric sounding observations carried out at Palmas (10.2°S, 48.2°W; located close to the magnetic equator) and Sao Jose dos Campos (23.2°S, 45.9°W; located under the southern crest of equatorial ionospheric anomaly, close to Brazopolis), Brazil, to study the day‐to‐day variability in the development of ionospheric plasma bubbles during both geomagnetically disturbed and quiet periods in September–October 2002. Also, we present simultaneous complementary phase fluctuation (ROT) data obtained from the global position system (GPS) meridional chain operated by the Brazilian Institute of Geography and Statistic (IBGE). On the three nights studied in the present investigation (one geomagnetically quiet and two geomagnetically disturbed), it has been observed that the geomagnetic disturbances, during this spring equinox period, have a strong effect on the generation and development of ionospheric plasma bubbles.

Effects observed in the Latin American sector ionospheric F region during the intense geomagnetic disturbances in the early part of November 2004

The Sun was very active in the early part of November 2004. During the period of 8–10 November 2004, intense geomagnetic disturbances with two superstorms were observed. In this paper, we have investigated the generation and suppression of equatorial ionospheric irregularities and the daytime changes in the F region electron density in the Latin American sector during the period of intense geomagnetic disturbances. We present the ionospheric sounding observations carried out at Manaus and Sao Jose dos Campos, Brazil, during this geomagnetically disturbed period. Also, GPS observations obtained from several stations in Brazil, Argentina, and St. Croix, U.S. Virgin Islands, during the disturbed period are presented. During the main phase of the first superstorm, around the prereversal enhancement time (night of 7–8 November), prompt penetration of electric field was observed and the presence of equatorial ionospheric irregularities was detected from St. Croix, U.S. Virgin Islands (in the northern hemisphere) to Bahia Blanca, Argentina (in the southern hemisphere). The ionospheric sounding observations at Manaus indicate inhibition of prereversal enhancement on the nights of 9–10 and 10–11 November, possibly due to the disturbed thermospheric winds or disturbance electric fields. Virtually no phase fluctuations on the nights of 9–10 and 10–11 November were observed in the Latin American sector. During the daytime on 8 November, the vertical total electron content (VTEC) observations show a negative storm phase at Porto Alegre (Brazil) and Bahia Blanca (Argentina). Again during the daytime on 10 November, the VTEC observations show a negative storm phase from Brasilia (Brazil) to Bahia Blanca. These negative storm phases are associated with a decrease in the O/N2 ratio. During the daytime on 9 November, the VTEC observations show a positive storm phase extending from St. Croix to Porto Alegre, and again on 10 November, VTEC observations show a positive storm phase. These positive storm phases observed are possibly due to changes in large‐scale wind circulation and an increase in the O/N2 ratio.

Observations of Ionospheric Irregularity Zonal Velocity in Hainan

根据中国海南富克(19.3°N,109.1°E)三点GPS观测系统2007年3月至11月的观测数据,利用互相关方法分析了三站闪烁信号的时间延迟,得出了不规则体纬向漂移的基本特征.在中国海南地区,闪烁主要发生在春秋季节,夜间不规则体的纬向漂移速度以东向为主,大小在50～150 m/s之间;平均东向漂移速度随时间呈下降趋势.另外,在闪烁刚发生时,不规则体纬向速度起伏较大,这可能与不规则体的随机起伏以及等离子体泡产生时垂直速度较大有关.中国海南地区不规则体纬向漂移速度的这些基本特征与低纬其他地区的测量结果较为一致.

Latitudinal and temporal variation of equatorial ionospheric irregularities determined from GPS scintillation observations

Abstract. In this paper we investigate the spatial and temporal distribution of ionospheric irregularities in the South-East Asian longitude sector by analysing the scintillation of GPS signals received at ground based stations. The data used for this work were collected during 1998 to 2002 with our regional network of GPS based scintillation monitors (Thomas et al., 2001). The aim of this analysis is to better understand the morphology and climatology of ionospheric irregularities in our region. Interesting differences between the north and south anomalies are noted when we examine the latitudinal–temporal variation of irregularity activity. We investigate the possible causes of these differences and note that variations in the irregularity seeding mechanism and in the background ionosphere at the two anomalies could be important. Phase screen diffraction theory is used to analyse our scintillation data and we note problems with this theory for ray-paths with large off-zenith angles. The height of the phase screen is another important issue. We discuss the implications for models which rely on phase screen diffraction theory.

Equatorial F-region plasma depletion drifts: latitudinal and seasonal variations

Abstract. The equatorial ionospheric irregularities have been observed in the past few years by different techniques (e.g. ground-based radar, digisonde, GPS, optical instruments, in situ satellite and rocket instrumentation), and its time evolution and propagation characteristics can be used to study important aspects of ionospheric dynamics and thermosphere-ionosphere coupling. At present, one of the most powerful optical techniques to study the large-scale ionospheric irregularities is the all-sky imaging photometer system, which normally measures the strong F-region nightglow 630 nm emission from atomic oxygen. The monochromatic OI 630 nm emission images usually show quasi-north-south magnetic field-aligned intensity depletion bands, which are the bottomside optical signatures of large-scale F-region plasma irregularities (also called plasma bubbles). The zonal drift velocities of the plasma bubbles can be inferred from the space-time displacement of the dark structures (low intensity regions) seen on the images. In this study, images obtained with an all-sky imaging photometer, using the OI 630 nm nightglow emission, from Cachoeira Paulista (22.7° S, 45° W, 15.8° S dip latitude), Brazil, have been used to determine the nocturnal monthly and latitudinal variation characteristics of the zonal plasma bubble drift velocities in the low latitude (16.7° S to 28.7° S) region. The east and west walls of the plasma bubble show a different evolution with time. The method used here is based on the western wall of the bubble, which presents a more stable behavior. Also, the observed zonal plasma bubble drift velocities are compared with the thermospheric zonal neutral wind velocities obtained from the HWM-90 model (Hedin et al., 1991) to investigate the thermosphere-ionosphere coupling. Salient features from this study are presented and discussed.Key words. Ionosphere (ionosphere-atmosphere interactions; ionospheric irregularities; instruments and techniques)

Magnetospheric disturbance induced equatorial plasma bubble development and dynamics: A case study in Brazilian sector

Equatorial ionospheric plasma bubble irregularity development and dynamics during the major magnetospheric storm of 26 August 1998 are investigated using the data collected by a multistation and multi‐instrument diagnostic network operated at equatorial and low latitude sites in Brazil, and auroral electrojet activity (AU/AL), IMF, and Dst indices. A magnetospheric disturbance onset in the morning of 26 August 1998 was initiated by a solar wind shock and associated IMF Bz polarity reversals and ssc that were soon followed by a succession of substorm‐like auroral electrojet (AE) intensifications and Dst development. An IMF Bz southward turning and associated AE intensifications in the Brazilian dusk sector produced intense prompt penetration eastward electric field that caused large F region vertical drift and consequently the developments of intense postsunset equatorial anomaly and a series of intense plasma bubbles, the latter event lasting the entire night, as observed by digital ionosondes at São Luís (2.33°S, 315.8°E, dip angle: −.5°) and Fortaleza (3.9°S, 321.55°W, dip angle: −9°) and an all‐sky imager, two scanning photometers, and a Digisonde at the low‐latitude site Cachoeira Paulista (22.6°S, 315°E; dip angle: −28°). A notable aspect of the dynamics of the bubbles was their initially very low eastward drift velocity which turned into steadily increasing westward velocity that lasted till early morning hours. The results show for the first time a relationship between the zonal drift velocities of optically observed large‐scale bubbles (tens to hundreds of kilometers) and that of the smaller scale (kilometer sizes) structures as observed by a digital ionosonde. The results point to the dominant role of a disturbance dynamo associated westward thermospheric wind to maintain the plasma irregularity drift increasingly westward going into postmidnight hours. As an important finding, the results further show that significant contribution to the westward plasma bubble irregularity drift, normally attributed to disturbance dynamo effect, could arise from prompt penetration disturbance zonal electric field, in the course of a disturbance sequence lasting several hours. Such effect is attributed to Hall electric field arising from the primary disturbance zonal electric field, under enhanced nighttime ionospheric conductivities produced possibly by storm associated particle precipitation, in the Brazilian longitude sector in agreement with recent evidences [Abdu et al., 1998b].

Studies of equatorial F-region depletions and dynamics using multiple wavelength nightglow imaging

A charge coupled device based all-sky imaging system with 180° field of view has been operating at Kolhapur (16.8°N, 74.2°E, dip lat. 10.6°N) in India on clear moonless nights to study the dynamics of equatorial ionospheric irregularities by examining the images of three oxygen lines at OI 630, 557.7 and 777.4 nm . The all-sky camera offers broad and instantaneous coverage of the movement of large-scale size ionospheric irregularities (depletions or bubbles) over a region of 1800 km at about 300 km altitude. The location of the station is important as the single image at a given instant maps the irregularities over a large area extending from the geomagnetic equator to the equatorial ionization anomaly region. Several characteristics of irregularities discussed in this report were observed during ISTEP campaign periods conducted in March–April of 1998 and 1999. One example of the signature of simultaneous depletions observed in OI 630, 557.7 and 777.4 nm images is reported. Also, the signature of midnight temperature maximum (MTM) in OI 630 nm airglow has been presented. The tilt of the MTM structure with magnetic meridian was found to change during the course of its movement towards north. The eastward plasma drift speeds ranging from 0 to 289 m/ s computed from bubble movement matches well with the drift speeds computed from VHF scintillation experiments and Jicamarca radar observations.

First in situ observations of equatorial ionospheric bubbles by Indian satellite SROSS‐C2 and simultaneous multisatellite scintillations

The first observation of equatorial ionospheric irregularities by RPA probe of the Indian low Earth orbiting satellite SROSS‐C2 is presented in this paper. Amplitude scintillations of medium Earth orbiting Global Positioning System (GPS) satellites and geostationary FLEETSATCOM (244 MHz, 73°E) and INMARSAT (1.5 GHz, 65°E) signals recorded simultaneously at Calcutta (lat: 22.97° N, long: 88.50°E geographic; dip: 32°N) are used for a coordinated study of equatorial F region irregularities in the Indian zone. Cases of ionospheric irregularities identified from the SROSS‐C2 records obtained during the initial one‐and‐a‐half years since its launch in May 1994 have been analyzed. Some events of in situ ion density irregularities are compared with scintillations simultaneously observed on the transionospheric satellite links. Intense bite‐outs of ion density (maximum relative irregularity amplitude ΔN/N ∼ 65%) were detected on one occasion (October 29, 1994) coupled with deep fadings (S4 ∼ 1 at VHF, ∼0.52 at L‐band, and ∼0.69 at GPS L1 frequency) on ground‐based satellite links. An estimate of scintillation indices from the observed in situ density deviations compares well with the ground‐based measurements. The development of intense equatorial bubbles even on a day like October 29, 1994, under low solar activity conditions, may be attributed to a prompt penetration of magnetospheric electric field equatorwards during the main phase of a magnetic storm in progress [maximum negative excursion of Dst ∼ −127 nT at 1600UT (2100MLT) with a dDst/dt rate −37 nT/hr at 1300–1400UT (1800–1900MLT)]. The drift velocity and spatial extent of these irregularities have been estimated from ground‐based observations.

Equatorial spread F found in 5577 Å and 6300 Å airglow observations from Hawaii

We have searched for F region ionospheric disturbances in a database of airglow observations from Mt. Haleakala, Hawaii. The airglow scan observations were made with narrowband filters of OI 5577 Å and OI 6300 Å for 164 nights in the period of April 1965 through October 1968. The OI 6300 emissions are produced mostly via recombination of O2+ in the F region, whereas the OI 5577 emissions are generated both in the E and F regions. The database has unique significance, since it covers more than three years of observations in the 1960s when there was no other program that systematically monitored ionospheric airglows. We find disturbances in the OI airglow brightness, from which we constructed projected maps of airglow distributions at the F region altitude. The projected maps clearly show wavelike disturbances whose crests are mostly aligned along a north–south direction, structures seen even in the raw scan data. The disturbances were found exclusively in the southerly azimuth toward the equator, implying that they are one of the equatorial ionospheric irregularities, known as equatorial spread F (ESF). The ESF disturbances occurred 19 nights out of a total of 161 nights of azimuth‐scanning observations with a maximum in September. The disturbances seem to arise regardless of geomagnetic activities. The seasonal variation of the occurrence is consistent with recent analyses of satellite data in which ESF is suggested to be caused by gravity waves from the troposphere, where they are generated by strong convection in intertropical convergence zone.

Solar cycle effects on large scale equatorial F-region plasma depletions

In recent years, scientists have used wide-angle imaging observations of F-region nightglow emissions (e.g., OI630 nm and OI777.4 nm) to map the F-region equatorial ionospheric irregularity structures, characterized by large scale plasma depletions, generally known as transequatorial plasma bubbles. In Brazil, an OI630 nm all-sky imaging system has been in operation at Cachoeira Paulista (22.7°S, 45.0°W) since March 1987. This has provided an extensive data-base of the OI630 nm all-sky imaging observations in the equatorial ionospheric anomaly region. In this paper, the morphology of large scale equatorial F-region plasma depletions under sunspot maximum and minimum conditions is presented and discussed. It has been observed that the occurrences of plasma depleted regions are relatively lesser during low solar activity than during high solar activity. The seasonal occurrence pattern is fairly similar for high and low solar activities.

OI 630 nm imaging observations of equatorial plasma depletions at 16° S dip latitude

Equatorial ionospheric irregularities in the F-layer have been the subject of intensive experimental and theoretical investigations during recent years. The class or irregularities which continues to receive much attention is characterized by large scale plasma depletions, generally referred to as ionospheric plumes and bubbles. The OI 630.0 nm F-region night-glow emissions arising from recombination processes can be used to observe the dynamics of transequatorial ionospheric plasma bubbles and smaller scale plasma irregularities. In a collaborative project between the Center for Space Physics of Boston University and Brazil's National Institute for Space Research (INPE), an all-sky imaging system was operated at Cachoeira Paulista (22.7° S, 45.0° W, dip latitude 15.8° S), between March 1987 and October 1991. In addition to the imager, photometer and VHP polarimeter observations were conducted at Cachoeira Paulista, with ionospheric soundings carried out at both C. Paulista and Fortaleza, the latter at 3.9° S, 38.4° W, 3.7° S dip latitude. For this longitude, the observed seasonal variation of the airglow depletions shows a maximum from October through March and a very low occurrence of airglow depletions from April through September. This long series of OI 630.0 nm imaging observations has permitted us to determine that when there are extended plumes, the altitudes affected over the magnetic equator often exceed 1500 km and probably exceed 2500 km at times, the maximum projection that can be seen from Cachoeira Paulista. This holds true even during years of low solar flux.

Periodic amplitude variations as precursors of plumes of equatorial ionospheric irregularities

Periodic amplitude fluctuations of VHF signals from a geostationary satellite monitored from near the magnetic equator have been observed in the evening hours as precursors of strong Rayleigh fading associated with plumes of irregularities. These periodic fluctuations called “amplitude waves” exhibit amplitude changes of only 1 to 2 dB and have been observed for up to 30 minutes before the onset of strong scintillations. Individual fades are correlated over distances of at least 120 km in the magnetic eastwest direction. The velocity of these wavelike disturbances has been found to be approximately 140 ms −1 eastward with a corresponding wavelength of 25 km. No wavelike behavior of Faraday rotation, a measure of the background changes in TEC, was observed during these times. Several mechanisms are examined as the cause of these amplitude waves; however, none was found to be completely satisfactory in explaining the observations.

Equatorial ionospheric irregularities produced by the Brazilian Ionospheric Modification Experiment (BIME)

On two separate evenings in September 1982, rockets were launched into the bottomside equatorial F2 region off the coast of Natal, Brazil, to inject chemicals, consisting of mainly H2O and CO2, to create a hole in ionization. The chemicals were injected near the height where the density gradient was steepest, and at a time when the F2 region was rising rapidly, to see whether plasma bubble irregularities could be generated from instabilities triggered by the ionization hole. On both occasions, hole‐induced depletions in total electron content (TEC) of more than 1016 el/m² were observed over horizontal distances of at least 60 km from the chemical injection point. The eastward drifts of these artificial depletions were observed by the time difference in the TEC features observed at various TEC monitoring stations, and from the changing range of oblique ionosonde echoes observed by an ionosonde located 300 km magnetically east of the chemical release point. Their subsequent evolution into plasma bubble irregularities was demonstrated from the observations of spread F echoes, strong‐amplitude scintillation, and TEC depletion at distances of from 300 to 500 km eastward of the release points. The fact that similar behavior of the ionosphere was observed during the evenings of both rocket chemical releases, and on no other nights of the campaign, is strong evidence of successful artificial generation of bubble irregularities by chemical injection into the bottomside F2 region.

Short‐term magnetic field alignment variations of equatorial ionospheric irregularities

The ionospheric irregularities that cause equatorial scintillation are elongated along the north‐south magnetic field lines. During a 1981 field campaign at Ascension Island, 250‐MHz receivers were spaced from 300 m to 1.6 km along the field lines, and the signals received from the Marisat satellite were cross correlated. Data collected during eight nights of fading showed a linear relationship between fading rate and cross correlation. The alignment of the antennas was adjusted to give a zero time lag between the widely spaced receivers with a measurement accuracy of 0.03 s. Since the average irregularity velocity was 125 m/s, this time accuracy translated to an angular measurement accuracy of 0.1°. During a 4‐hour period of nightly fading, occasional differences in time of arrival were noted that corresponded to a tilt in the north‐south alignment of ±1°. Data from several nights of fading were analyzed, and each night exhibited the same variance in the north‐south irregularity alignment. It is postulated that the shift in the measured peak correlation may have been caused by patches of irregularities at different altitudes where the magnetic field lines have a slightly different direction.

Equatorial ionospheric plasma bubble irregularity occurrence and zonal velocities under quiet and disturbed conditions, from Polarimeter observations

Amplitude scintillation and Faraday rotation angle fluctuations of satellite VHF beacons registered by electronic polarimeters operated at two stations spaced 110 km apart in the magnetic east‐west direction namely, Cachoeira Paulista, 22°41′S, 45°W, dip angle of −26°, and São José dos Campos, 23°12′S, 45°51′W, dip angle of −26°, in Brazil are analyzed for equinoctial and summer months of 1982–1983 to determine the irregularity patch occurrence characteristics and their zonal velocities under varying levels of magnetic disturbances. The scintillation patch occurrences and total electron content fluctuations representing plasma depletion structures decrease during premidnight hours and are enhanced in the postmidnight period. The occurrences often extend to morning hours, with increasing magnetic activity index. While the night events are associated with eastward irregularity patch motion, the morning events have westward velocities (the reversal) occurring around 0400 LT, reflecting the morning velocity reversal in the F region bulk plasma motion reported earlier from Jicamarca radar measurements. The westward plasma bubble motions reported here represent the first observation of this kind. Another important finding from the present investigation is a clear tendency for the zonal plasma motion, namely, the equatorial meridional electric field, to become enhanced mainly in the premidnight hours immediately following strong magnetic activity.

Scintillation results from ionospheric modification experiment

During September 1982 a group of U.S., West German, and Brazilian organizations evaluated the trigger mechanism of equatorial ionospheric bubbles in an ionospheric modification experiment in Brazil. The experiment involved the release of two barium chemical packages at an unstable point in the nighttime F layer. At this point, they would trigger a bubble of low ion density to rise into the middle and upper F regions. Following the release of the barium, an aircraft equipped with a UHF satellite receiver was flown in the shadow of the bubble. It mapped the growth of the resulting irregularities from their initial appearance until they grew to a width of 100 km or more. The experiment demonstrated that a bubble could be triggered by proper placement of the barium ions and that the resultant irregularities would have the appearance of naturally triggered equatorial ionospheric irregularities.

Statistical characterizations of equatorial scintillation in the Asian region

The morphological aspect of gigahertz equatorial scintillations in the Asian region has been reported. As a follow‐up, this paper studies the statistical aspect of equatorial scintillations in the same region. Power spectra of prominent scintillation events collected over a 16‐month period during the solar maximum years in sunspot cycle 21 were analyzed. The slope ν of the spectrum at the high‐frequency end was analyzed with respect to the strength of scintillation as indicated by scintillation index S4. The values of S4 were compared with theoretical calculations based on either phase screen formulation or Rytov approximation. These comparisons were used to assess ionospheric parameters such as the height and thickness of the irregularities layers, rms fluctuation of the total electron content, and the axial ratio of the irregularities. Autocorrelation calculation provided a characteristic coherent interval τ. The study suggests that equatorial ionospheric irregularities in the F region, with subkilometer sizes in the evening hours after local sunset, are the main cause of the gigahertz scintillations observed. The spectra of the irregularities appear to be the power law type, with spectral indices p that generally have values greater than 4. The characterizations of S4, ν, and τ for both the Pacific and Indian Ocean region links provide an important basis for improved modeling of ionospheric scintillation phenomena in the Asian region.

Observations and modeling of multi‐frequency VHF and GHz scintillations in the equatorial region

Amplitude scintillation spectra and statistics obtained simultaneously at VHF (257.55 MHz), L band (1541.5 MHz), and C band (3945.5 MHz) from the MARISAT communications satellite (15°W) are presented. The measurements were made at Ascension Island (7°58′S, 14°25′W, 30°S DIP) over a high elevation propagation path within a few degrees of the magnetic meridian plane. The multi‐frequency data and the unique propagation geometry enable us to study the structures of the equatorial ionospheric irregularities closely. Numerical simulation is used to model a specific event. Comparisons between the modeling results and the observations indicate that the data are consistent with the recently measured (in situ) two‐component power law irregularity spectrum with a shallow large‐scale regime and a steeper small‐scale regime, and a spectral break at about 1‐km scale size.