Diurnal Anisotropy Research Articles

Anomaly in the Diurnal Anisotropy During the Transition Period of Solar Cycles 24 and 25

Anomaly in the Diurnal Anisotropy During the Transition Period of Solar Cycles 24 and 25

Satellite-based daytime urban thermal anisotropy: A comparison of 25 global cities

Remotely sensed land surface temperature (LST) has become one of primary observational sources for urban climate research. However, LST is subject to considerable angular variation, particularly in urban areas caused by a large variation of sensor viewing angles and non-isothermal façades of 3D urban structures. Anisotropy varies seasonally and diurnally, and is sensitive to sensor viewing geometry, solar angles, and urban surface properties. Current studies focus on urban thermal anisotropy of individual cities, comparative study and the resultant driving factor analysis are still lacking. To bridge this critical knowledge gap, a systematic comparative assessment of satellite-based urban thermal anisotropy is required. Here, we modify and validate a satellite-based anisotropy estimation approach and compare the seasonal and diurnal anisotropy variations over 25 global cities to answer the following research questions: 1) How do the anisotropy magnitude and profile shapes vary among different cities? 2) What are the factors influencing the anisotropy? The results show that off-nadir observations are up to 3.5 K cooler on average than nadir LST during summer whereas the effective anisotropy ranges between 2.1 K and 5.8 K for all cities. Winter anisotropy is much smaller (∼ 1 K), where the warm or cold bias is determined by solar zenith angle (morning/afternoon overpasses), and its bias magnitude is linearly proportional to the sensor zenith angle. Urban thermal anisotropy is partly influenced by geographic location (e.g., latitude, longitude), urban vegetation (measured by NDVI) and urban geometry (building height-street width ratio, h/w), where low latitude cities or cities with less vegetation tend to have a greater angular variation in summer, and the anisotropy tends to increase with more complex urban morphology (i.e., higher building aspect ratio). High vegetation cover makes the anisotropy distribution shape more asymmetric along the cross-track direction (or sensor zenith angle). Our findings help understand the seasonal and diurnal impact of anisotropy for wide-swath LST applications in cities and offers insights for addressing anisotropy in future satellite products.

Analysis of Ground Level Enhancement Events of 1989 September 29; 2001 April 15 and 2005 January 20

We present the results of analyses of the ground level enhancements (GLEs) of cosmic ray (CR) events on 1989 September 29; 2001 April 15 and 2005 January 20. This involves examination of hourly raw CR counts of an array of neutron monitors (NMs) spread across different geographical latitudes and longitudes. Using awk script and computer codes implemented in R software, the pressure corrected raw data plots of the NMs were grouped into low-, mid- and high-latitudes. The results show both similarities and differences in the structural patterns of the GLE signals. In an attempt to explain why the CR count during the decay phase of GLEs is always higher than the count before peak, we interpreted all counts prior to the peak as coming from direct solar neutrons and those in the decay phase including the peak as coming from secondary CR neutrons generated by the interactions of primary CRs with the atoms and molecules in the atmosphere. We identified NMs that detected these primary neutrons and found that they are close in longitude. Previous authors seemingly identified these two species as impulsive and gradual events. Although there are a number of unexplained manifestations of GLE signals, some of the results suggest that geomagnetic rigidity effectively determines the intensity of CRs at low- and mid-latitudes. Its impact is apparently insignificant in high-latitude regions. Nevertheless, the results presented should be validated before making any firm statements. Principally, the contributions of the ever-present and intractable CR diurnal anisotropies to GLE signals should be accounted for in future work.

Testing the Empirical Relationship between Forbush Decreases and Cosmic Ray Diurnal Anisotropy

The abrupt aperiodic modulation of cosmic ray (CR) flux intensity, often referred to as Forbush decrease (FD), plays a significant role in our understanding of the Sun–Earth electrodynamics. Accurate and precise determinations of FD magnitude and timing are among the intractable problems in FD-based analysis. FD identification is complicated by CR diurnal anisotropy. CR anisotropy can increase or reduce the number and amplitude of FDs. It is therefore important to remove its contributions from CR raw data before FD identification. Recently, an attempt was made, using a combination of the Fourier transform technique and FD-location machine, to address this. Thus, two FD catalogs and amplitude diurnal variation (ADV) were calculated from filtered (FD1 and ADV) and raw (FD2) CR data. In the current work, we test the empirical relationship between FD1, FD2, ADV and solar-geophysical characteristics. Our analysis shows that two types of magnetic fields - interplanetary and geomagnetic (Dst) - govern the evolution of CR flux intensity reductions.

Diurnal anisotropy of polar neutron monitors: Dome C looks poleward

Galactic cosmic rays (GCR) exhibit a small local anisotropy observed as diurnal variability of ground-based neutron monitor (NM) count rates. Since the asymptotic directions of various NMs are different, their ability to observe the GCR diurnal variation also varies. Here we show that the Dome C (DOMC) NM is hardly sensitive to the diurnal variation, with its amplitude being 0.03%, in contrast to other polar NMs whose sensitivity to the diurnal variability ranges from 0.16 to 0.4%. We argue that this is related to the fact that DOMC NM has a narrow asymptotic-direction cone looking nearly to the South pole (geographic latitude above 75°). This makes the DOMC NM a unique detector being the only existing NM accepting cosmic-ray particles originating from the off-equatorial region. This is important for detailed studies of cosmic-ray transport in the vicinity of Earth, specifically for anisotropic solar energetic particle events.

Amplitude of the Usual Cosmic Ray Diurnal and Enhanced Anisotropies: Implications for the Observed Magnitude, Timing, and Ranking of Forbush Decreases

Abstract Cosmic ray (CR) diurnal anisotropy and Forbush decreases (FDs), as well as the relationship between them, have received considerable study. Several astrophysicists have focused on the speculated impact of FDs on the amplitude of the diurnal CR variations on different timescales. In an attempt to disentangle the contribution of FDs, days of Forbush events are traditionally excluded while calculating the diurnal amplitude in individual neutron monitor (NM) data. But the implications of CR diurnal anisotropy on the magnitude of FDs are rarely investigated in detail. Recently, an effort was made, using a combination of Fast Fourier transform and FD-location algorithms, to account for the contribution of CR diurnal anisotropy on the number, magnitude, and timing of FDs. With some technical advancements, the efficiency of the software is tested in the current work using CR data measured by the oldest CR observatory (Climax) from 1953 to 2006. We find strong and statistically significant correlations between FD magnitude calculated from raw and Fourier transformed CR data and the amplitude of diurnal anisotropy. The relationship is stronger in the case of the Fourier transformed signal, lending credence to the idea that CR anisotropy is a part of Forbush events. In order to validate the observed relationship between the amplitude of CR diurnal variations, magnitude, and timing of FDs, large volumes of data from the Moscow, McMurdo, and Potchefstroom NMs were also analyzed. The significant CR intensity decreases (FD magnitude >5%) at the four stations were ranked according to their magnitudes.

Amplitude of the Observational Forbush Decreases in the Presence of Cosmic Ray Diurnal Anisotropy During High Solar Activity in 1972

Amplitude of the Observational Forbush Decreases in the Presence of Cosmic Ray Diurnal Anisotropy During High Solar Activity in 1972

Galactic cosmic ray transport in the heliosphere: Forbush ion chamber data analysis

The study of ratio α (=λ⊥/λ||) of galactic cosmic ray (GCR) free paths normal (λ⊥) and parallel (λ||) to the mean interplanetary magnetic field (IMF) B is continued with annual mean GCR solar diurnal anisotropy (SDA) data of the Cheltenham/Fredericksburg shielded ion chamber (CH-IC) for 1936–1977 previously analyzed by Bieber and Chen (1991) but not corrected for atmospheric temperature effect. This study uses CH-IC data corrected for temperature effect to calculate α for the first time assuming negligible latitudinal density gradient. Solar activity dependence of α shown by Bieber and Chen (BC) remains qualitatively unchanged. Present GCR modulation theories do not provide guidance for α, its rigidity or solar cycle dependence. Flat heliosphere current sheet (HCS) and Master equations derived by Ahluwalia and Dorman (1997) are used for computing α and study its rigidity dependence and correlation with solar activity for positive (p) and negative (n) polarity intervals of B at 1 AU; B points outward/ inward from the Sun during p/n interval in Northern hemisphere, IMF polarity changes sign near sunspot number (SSN) maximum. CH-IC median rigidity of response (Rm) for GCR differential spectrum is 67 GV. We show that computed α is high for p- and low for n-intervals, also α is low near SSN maximum and high near SSN minimum when effective limiting GCR rigidity (Rc) for SDA is known to be <100 GV. The results are consistent with those reported earlier by Ahluwalia and Modzelewska (2020) for NagV-MT (Rm = 60 GV) for 1971–2017; they challenge validity of present conceptual understanding of GCR modulation processes in the heliosphere, pointing to a great need to develop an acceptable modulation theory to explain empirical results reported in this paper.

Automated detection of simultaneous/non-simultaneous Forbush decreases and the associated cosmic ray phenomena

The knowledge of the periodic diurnal anisotropy in the cosmic ray (CR) flux intensity as well as the enhanced anisotropy that might accompany Forbush events is not novel. Theoretical models and several methodological approaches such as Fourier techniques developed to predict the diurnal vectors started appearing in the literature since 1960s’. Consequently, the vectors have been successfully calculated on both annual, monthly and day to day basis. The amplitude and the phase of the unusually large diurnal wave trains that happen at the time of FDs have also been estimated by the method of Fourier analysis. However, direct application of Fourier transform algorithms to account for the enhanced CR diurnal anisotropies during the period of Forbush events is one of the open questions in CR research. Manual FD event identification technique, traditionally employed to select the short-term rapid depressions (Forbush decreases, FDs) in CR intensity is inefficient to handle the characteristic CR flux superposition tendencies. Identifications of FDs and the accompanying enhanced amplitude of the CR diurnal anisotropies are rarely investigated simultaneously. This poses a drawback on quantitative prediction of the impact of CR anisotropies on the detectable number of global and location dependent FDs as well as their magnitudes. A detailed and rigorous analysis involving simultaneous application of Fourier analysis to FDs and diurnal anisotropies, selections of FDs from Fourier transformed and raw CR data are attempted in the current work. The results are discussed with emphasis on simultaneity of FDs detected at different locations at Earth.

Galactic cosmic ray transport in the heliosphere: Study with muon data

We use the galactic cosmic ray (GCR) solar diurnal anisotropy (SDA) data of Nagoya vertical muon telescope (NagV-MT) to compute the ratio α of GCR free paths normal and parallel to mean interplanetary magnetic field (IMF) B. The values are compared to those computed earlier for the neutron monitors (NMs) of the global network for three sunspot cycles (21–23) and parts of the other two (19, 24). Current GCR modulation theories do not provide any guidance for α, its rigidity and solar cycle dependence. We use a flat heliosphere current sheet and the Master equations derived by Ahluwalia and Dorman [1997] for computing α and study its rigidity dependence and correlation with solar activity for positive (p) and negative (n) polarity intervals of B at 1 AU; B points outward/ inward from the sun during p/n interval in north hemisphere; IMF polarity changes sign near SSN maximum. For NagV-MT (Rm = 60 GV), Rm being the median value of response to GCR differential rigidity spectrum, we find that α is high for p- and low for n-intervals, compared to being nearly flat for NMs; α for NM2 (Rm = 25 GV) is larger than that for NM1 (Rm = 15 GV), implying an increase in α with GCR rigidity. The α values for NagV-MT are low near SSN maximum as is the case for NMs and high near SSN minimum, when effective limiting GCR rigidity (Rc) for SDA is known to be lower than 100 GV. The results challenge the validity of current conceptual understanding of GCR modulation processes in the heliosphere, pointing to a great need to develop an acceptable theory to explain the empirical results reported in this paper.

On the Solar Cycle Variation of the Solar Diurnal Anisotropy of Multi-TeV Cosmic-ray Intensity Observed with the Tibet Air Shower Array

We analyze the temporal variation of the solar diurnal anisotropy of the multi-TeV cosmic-ray intensity observed with the Tibet air shower array from 2000 to 2009, covering the maximum and minimum of the 23rd solar cycle. We comfirm that a remarkable additional anisotropy component is superposed on the Compton-Getting anisotropy at 4.0 TeV, while its amplitude decreases at higher energy regions. In constrast to the additional anisotropy reported by the Matsushiro experiment at 0.6 TeV, we find the residual component measured by Tibet at multi-TeV energies is consistent with being stable, with a fairly constant amplitude of 0.041% ± 0.003% and a phase at around 07.17 ± 00.16 local solar time at 4.0 TeV. This suggests the additional anisotropy observed by the Tibet experiment could result from mechanisms unrelated to solar activities.

Analysis of urban surface morphologic effects on diurnal thermal directional anisotropy

Remote thermal radiative observations over metropolitan areas are subject to an angular-dependent variation, known as the directional thermal anisotropy. The 3D urban surface morphology is one key factor in determining the magnitude and temporal variation of thermal anisotropy. This study uses 3D building data and the Town Energy Balance model (TEB) to explore the impact of morphological variability on diurnal anisotropy patterns, and quantifies errors introduced by a simplification of urban morphology as an array of evenly distributed uniform cubes. Results from the comparison of two representative urban districts in Brooklyn and midtown Manhattan, New York City reveal distinct diurnal anisotropy patterns. Daytime anisotropy varies more time-sensitively over the compact high-rise district of Manhattan, although the maximum effective anisotropy (the maximum contrast of directional anisotropy) is smaller than Brooklyn, which is related to a reduced contrast among wall temperatures. A stronger angular effect at night is found as the aspect-ratio increases. The anisotropy is further simulated at the Moderate Resolution Imaging Spectroradiometer (MODIS) overpass time and sensor-surface relative geometry over these two morphologic samples. The sensitivity test unravels that the effective anisotropy monotonically increases with a greater aspect ratio for MODIS nighttime overpasses, while the daytime pattern is more complex with a single- or double-peak distribution depending on the solar angle (or time of day). Finally, the variation of building height and size is important in determining the anisotropy from comparing simulations of a realistic 3D building model and a simplified urban morphology as cube array. The morphological simplification can lead to a higher discrepancy in these cases with a high aspect ratio or small sky view factor for both daytime and nighttime. The proposed 3D-computer-graphics approach is computationally affordable for the seen surface estimation and can be applied to IFOVs across a relatively large urban area. Its flexibility in integrating various levels of 3D urban surface complexity makes it a promising tool for correcting the urban thermal anisotropy from satellite observations in the future.

Latitudinal and longitudinal dependence of the cosmic ray diurnal anisotropy during 2001–2014

Abstract. The diurnal anisotropy of cosmic ray intensity for the time period 2001 to 2014 is studied, covering the maximum and the descending phase of solar cycle 23, the minimum between solar cycles 23 and 24, and the ascending phase and maximum of solar cycle 24. Cosmic ray intensity data from 11 neutron monitor stations located at different places around the Northern Hemisphere obtained from the high-resolution Neutron Monitor Database (NMDB) were used. Special software was developed for the calculations of the amplitude and the phase of the diurnal anisotropy vectors on annual and monthly basis using Fourier analysis and for the creation of the harmonic dial diagrams. The geomagnetic bending for each station was taken into account in our calculations determined from the asymptotic cones of each station via the Tsyganenko96 (Tsyganenko and Stern, 1996) magnetospheric model. From our analysis, it was resulted that there is a different behavior of the diurnal anisotropy vectors during the different phases of the solar cycles depending on the solar magnetic field polarity. The latitudinal and longitudinal distribution of the cosmic ray diurnal anisotropy was also examined by grouping the stations according to their geographic coordinates, and it was shown that diurnal variation is modulated not only by the latitude but also by the longitude of the stations. The diurnal anisotropy during strong events of solar and/or cosmic ray activity is discussed.

Solar cycle and 27-day variations of the diurnal anisotropy of cosmic rays during the solar cycle 23

The diurnal anisotropy of cosmic-ray intensity observed over the period 1997–2006, which coincides with the solar cycle 23, has been analyzed using cosmic ray data from Athens and Oulu neutron monitor stations. In this analysis it was observed that the time of the diurnal variation maximum shifted to earlier hours than the corotation direction from 1997 to 2000, where the polarity state of the magnetic field was positive ( $qA > 0$ ) and to later hours from 2001 to 2006 where the polarity state was negative ( $qA < 0$ ). Moreover the 27-day variation of the cosmic-ray diurnal anisotropy in connection with the 27-day variation of the interplanetary magnetic field has been studied. In this study six groups of ten Bartel rotations each one have been analyzed (2232–2241, 2249–2258, 2275–2284, 2286–2295, 2328–2337, 2356–2365). It is remarkable that the 27-day variation of cosmic- ray intensity is characterized by the well known sector structure and it is well correlated with the $B_{XY}$ component of the interplanetary magnetic field. These findings during the last solar cycle confirm once again the close relation of the diurnal variation of cosmic ray intensity and the interplanetary magnetic field.

Diurnal anisotropy of cosmic rays during intensive solar activity for the period 2001–2014

The diurnal variation of cosmic ray intensity, based on the records of two neutron monitor stations at Athens (Greece) and Oulu (Finland) for the time period 2001 to 2014, is studied. This period covers the maximum and the descending phase of the solar cycle 23, the minimum of the solar cycles 23/24 and the ascending phase of the solar cycle 24.These two stations differ in their geographic latitude and magnetic threshold rigidity. The amplitude and phase of the diurnal anisotropy vectors have been calculated on annual and monthly basis.From our analysis it is resulted that there is a different behaviour in the characteristics of the diurnal anisotropy during the different phases of the solar cycle, depended on the solar magnetic field polarity, but also during extreme events of solar activity, such as Ground Level Enhancements and cosmic ray events, such as Forbush decreases and magnetospheric events. These results may be useful to Space Weather forecasting and especially to Biomagnetic studies.

North–south component of galactic cosmic ray anisotropy at 1 AU

The galactic cosmic ray (GCR) solar diurnal anisotropy (SDA) may be represented by a vector (A; Ar, Aφ, Aθ) in a spherical polar coordinate system centered on the sun. We reported (elsewhere) the results of a detailed study of time variations of yearly radial (Ar) and east–west (Aφ) components recorded by the global network of the neutron monitors (NMs) with a long track record for 1963–2013, for four sunspot number (SSN) cycles (20–23) and the rising phase of cycle 24. A powerful new technique is used to compute and study time variations of the transverse component (Aθ) due to off-ecliptic GCR contributions, with the same NM data; GCR radial particle density gradient (Gr) drives all three components. The north–south anisotropy (Aθ) is computed from yearly NM data (Gϕ=0), a flat heliospheric current sheet (HCS) model and the concept of GCR isotropic hard sphere scattering in the solar wind plasma. Relationships to SSN, rigidity and solar polarity intervals are studied. For a positive (p-) polarity the solar magnetic field in the northern hemisphere points outward and GCRs drift from polar regions toward equatorial plane and out along HCS, setting up a symmetric gradient (Gθs) pointing away from HCS (there is a local GCR density minimum on HCS); for n-polarity interval Gθs points towards HCS (there is a local GCR density maximum on HCS). Also, there exists a heliospheric asymmetric density gradient (Gθa) perpendicular to the ecliptic plane, it is the main contributor to Aθ for the period of our analysis. This is the most interesting and significant insight.

Cosmic ray heliospheric transport study with neutron monitor data

AbstractDetermining transport coefficients for galactic cosmic ray (GCR) propagation in the turbulent interplanetary magnetic field (IMF) poses a fundamental challenge in modeling cosmic ray modulation processes. GCR scattering in the solar wind involves wave‐particle interaction, the waves being Alfven waves which propagate along the ambient field (B). Empirical values at 1 AU are determined for the components of the diffusion tensor for GCR propagation in the heliosphere using neutron monitor (NM) data. At high rigidities, particle density gradients and mean free paths at 1 AU in B can only be computed from the solar diurnal anisotropy (SDA) represented by a vector A (components Ar, Aϕ, and Aθ) in a heliospherical polar coordinate system. Long‐term changes in SDA components of NMs (with long track record and the median rigidity of response Rm ~ 20 GV) are used to compute yearly values of the transport coefficients for 1963–2013. We confirm the previously reported result that the product of the parallel (to B) mean free path (λ||) and radial density gradient (Gr) computed from NM data exhibits a weak Schwabe cycle (11y) but strong Hale magnetic cycle (22y) dependence. Its value is most depressed in solar activity minima for positive (p) polarity intervals (solar magnetic field in the Northern Hemisphere points outward from the Sun) when GCRs drift from the polar regions toward the helioequatorial plane and out along the heliospheric current sheet (HCS), setting up a symmetric gradient Gθs pointing away from HCS. Gr drives all SDA components and λ|| Gr contributes to the diffusive component (Ad) of the ecliptic plane anisotropy (A). GCR transport is commonly discussed in terms of an isotropic hard sphere scattering (also known as billiard‐ball scattering) in the solar wind plasma. We use it with a flat HCS model and the Ahluwalia‐Dorman master equations to compute the coefficients α (=λ⊥/λ∥) and ωτ (a measure of turbulence in the solar wind) and transport parameters λ||, λ⊥, Gr, Gθs, and an asymmetric gradient Gθa normal to the ecliptic plane. We study their dependence on rigidity (R), p/n intervals, sunspot numbers (SSNs), and solar wind parameters at 1 AU. λ|| exhibits a strong 22y dependence but Gr does not, explaining solar polarity dependence of λ|| Gr. The computed Gr values are an order of magnitude greater than those reported by our colleagues making an ad hoc assumption that α is low (0.01). At high rigidities, the drift contribution at 1 AU is small and unsteady. A new methodology is outlined to compute yearly GCR north‐south anisotropy (Aθ) from the data for a single detector sorted for p/n intervals. We show that Gθa is the main contributor to Aθ in the steady state, and Gθa is shown not correlated with the north‐south excess SSNs.

Effects of the Heliospheric Current Sheet on Trains of Enhanced Diurnal Variations in Galactic Cosmic Rays

We present recurrent trains of the enhanced diurnal anisotropy (DA) of Galactic cosmic rays (GCRs) during 2010 as observed by the Princess Sirindhorn Neutron Monitor at Doi Inthanon with a vertical cutoff rigidity of 16.8 GV. By investigating synoptic charts of the heliospheric current sheet (HCS) and the trains of enhanced DA, we found some consistency between them. During most time periods when recurrent trains of enhanced DA occurred, the DAs were noticeably enhanced more during the away sector than during the toward sector of the interplanetary magnetic field $\mathbf{B}$ . We interpret these temporal features in terms of a $\mathbf{B}\times\nabla n$ gradient anisotropy, which is associated with latitudinal gradients of the GCR density ( $n$ ) derived from the perpendicular component of DA in the solar wind frame. The gradual changes of DA in the trains in turn are associated with the gradual change in the latitudinal gradient of the GCR density along the HCS. We found that the north–south asymmetry (offset) of the HCS is responsible for the asymmetric recurrent modulation of the GCR intensity and suggest that particle drift along the wavy HCS and the adiabatic loss can cause the recurrent modulation, which is related to a local latitudinal gradient in the GCR density. The causes of the latitudinal gradient are discussed in light of current theories and observations.

Study of Higher Harmonics of Cosmic Ray Intensity on Quiet Days at Deep River Neutron Monitoring Station

The cosmic ray (CR) intensity data recorded with Deep River Neutron Monitoring Station have been investigated on quietest days (QD) for higher harmonics of daily variation during solar cycle 21 and 22. It has been observed that inspite of the abrupt change in the amplitude and phase of tri-diurnal and quart diurnal anisotropy in CR intensity, the amplitude of third harmonic is quite significant throughout the period of investigation with larger amplitude during the year 1980 and 1985. Thus, tri-diurnal anisotropy clearly shows 11 year variation at mid latitude neutron monitoring station.

Effect of Polarity Reversal on Third Harmonics of Cosmic Ray Intensity on Quiet Days

The cosmic ray (CR) intensity data recorded with Goose Bay Neutron Monitoring Station have been investigated on 60 quietest days (QD) in a year for studying the variation in tri diurnal anisotropy during solar cycle 21 and 22. It has been observed that in spite of abrupt change in the amplitude and phase of tri-diurnal anisotropy in CR intensity, the amplitude is quite significant throughout period of investigation with small amplitude during 1981 and 1990. Further, the phase shift to earlier hours during 1990-91 showing the dependence on the polarity of solar magnetic field, which is attributed to drift effect. Thus, the tri-diurnal anisotropy clearly shows 11 year type variation at mid-latitude neutron monitoring station.