High Electron Flux Research Articles

Ion Densities in the Nightside Ionosphere of Mars: Effects of Electron Impact Ionization.

We use observations from the Mars Atmosphere and Volatile EvolutioN(MAVEN) mission to show how superthermal electron fluxes and crustal magnetic fields affect ion densities in the nightside ionosphere of Mars. We find that, due to electron impact ionization, high electron fluxes significantly increase the , O+, and densities below 200 km, but only modestly increase the NO+ density. High electron fluxes also produce distinct peaks in the , O+, and altitude profiles. We also find that superthermal electron fluxes are smaller near strong crustal magnetic fields. Consequently, nightside ion densities are also smaller near strong crustal fields because they decay without being replenished by electron impact ionization. Furthermore, the ratio is enhanced near strong crustal fields because, in the absence of electron impact ionization, is converted into NO+ and not replenished. Our results show that electron impact ionization is a significant source of , O+, and in the nightside ionosphere of Mars.

Continuous In-Flight Synthesis for On-Demand Delivery of Ligand-Free Colloidal Gold Nanoparticles.

We demonstrate an entirely new method of nanoparticle chemical synthesis based on liquid droplet irradiation with ultralow (<0.1 eV) energy electrons. While nanoparticle formation via high energy radiolysis or transmission electron microscopy-based electron bombardment is well-understood, we have developed a source of electrons with energies close to thermal which leads to a number of important and unique benefits. The charged species, including the growing nanoparticles, are held in an ultrathin surface reaction zone which enables extremely rapid precursor reduction. In a proof-of-principle demonstration, we obtain small-diameter Au nanoparticles (∼4 nm) with tight control of polydispersity, in under 150 μs. The precursor was almost completely reduced in this period, and the resultant nanoparticles were water-soluble and free of surfactant or additional ligand chemistry. Nanoparticle synthesis rates within the droplets were many orders of magnitude greater than equivalent rates reported for radiolysis, electron beam irradiation, or colloidal chemical synthesis where reaction times vary from seconds to hours. In our device, a stream of precursor loaded microdroplets, ∼15 μm in diameter, were transported rapidly through a cold atmospheric pressure plasma with a high charge concentration. A high electron flux, electron and nanoparticle confinement at the surface of the droplet, and the picoliter reactor volume are thought to be responsible for the remarkable enhancement in nanoparticle synthesis rates. While this approach exhibits considerable potential for scale-up of synthesis rates, it also offers the more immediate prospect of continuous on-demand delivery of high-quality nanomaterials directly to their point of use by avoiding the necessity of collection, recovery, and purification. A range of new applications can be envisaged, from theranostics and biomedical imaging in tissue to inline catalyst production for pollution remediation in automobiles.

Dynamic responses of the Earth's radiation belts during periods of solar wind dynamic pressure pulse based on normalized superposed epoch analysis

AbstractUsing the electron flux measurements obtained from five satellites (GOES 15 and POES 15, 16, 18, and 19), we investigate the flux variations of radiation belt electrons during forty solar wind dynamic pressure pulses identified between September 2012 and December 2014. By utilizing the mean duration of the pressure pulses as the epoch timeline and stretching or compressing the time phases of individual events to normalize the duration by means of linear interpolation, we have performed normalized superposed epoch analysis to evaluate the dynamic responses of radiation belt energetic electrons corresponding to various groups of solar wind and magnetospheric conditions in association with solar wind dynamic pressure pulses. Our results indicate that by adopting the timeline normalization we can reproduce the typical response of the electron radiation belts to pressure pulses. Radiation belt electron fluxes exhibit large depletions right after the Pdyn peak during the periods of northward interplanetary magnetic field (IMF) Bz and are more likely to occur during the Pdyn pulse under southward IMF Bz conditions. For the pulse events with large negative values of (Dst)min, radiation belt electrons respond in a manner similar to those with southward IMF Bz, and the corresponding postpulse recovery can extend to L ~ 3 and exceed the prepulse flux levels. Triggered by the solar wind pressure enhancements, deeper earthward magnetopause erosion provides favorable conditions for the prompt electron flux dropouts that extend down to L ~ 5, and the pressure pulses with longer duration tend to produce quicker and stronger electron flux decay. In addition, the events with high electron fluxes before the Pdyn pulse tend to experience more severe electron flux dropouts during the course of the pulse, while the largest rate of electron flux increase before and after the pulse occurs under the preconditioned low electron fluxes. These new results help us understand how electron fluxes respond to solar wind dynamic pressure pulses and how these responses depend on the solar wind and geomagnetic conditions and on the preconditions in the electron radiation belts.

Design of a high-flux instrument for ultrafast electron diffraction and microscopy

We present the design and optimization of a new instrument for ultrafast electron diffraction and imaging. The proposed instrument merges the high peak current and relativistic electron energies of radio-frequency guns, with the high average electron flux of static electron microscopes, extending the beam parameter space achievable with relativistic electrons by many orders of magnitude. An immediate consequence of this work is a broader range of accessible science by using electron probes, enabling techniques as femtosecond nano-diffraction and coherent diffraction imaging, and paving the way to direct observation of ultrafast dynamics in complex and isolated samples, from nanocrystals, to nano/micro droplets and organic molecules.

Analysis of plasmaspheric hiss wave amplitudes inferred from low‐altitude POES electron data: Validation with conjunctive Van Allen Probes observations

AbstractPlasmaspheric hiss plays an important role in controlling the overall structure and dynamics of the Earth's radiation belts. The interaction of plasmaspheric hiss with radiation belt electrons is commonly evaluated using diffusion codes, which rely on statistical models of wave observations that may not accurately reproduce the instantaneous global wave distribution or the limited in situ satellite wave measurements. This paper evaluates the performance and limitations of a novel technique capable of inferring wave amplitudes from low‐altitude electron flux observations from the Polar Orbiting Environmental Satellites (POES), which provide extensive coverage in shell and magnetic local time (MLT). We found that, within its limitations, this technique could potentially be used to build a dynamic global model of the plasmaspheric hiss wave intensity. The technique is validated by analyzing the conjunctions between the POES spacecraft and the Van Allen Probes from September 2012 to June 2014. The technique performs well for moderate‐to‐strong hiss activity (≥30 pT) with sufficiently high electron fluxes. The main source of these limitations is the number of counts of energetic electrons measured by the POES spacecraft capable of resonating with hiss waves. For moderate‐to‐strong hiss events, the results show that the wave amplitudes from the EMFISIS instruments on board the Van Allen Probes are well reproduced by the POES technique, which provides more consistent estimates than the parameterized statistical hiss wave model based on CRRES data.

Properties and origin of subproton‐scale magnetic holes in the terrestrial plasma sheet

AbstractElectron‐scale magnetic depressions in the terrestrial plasma sheet are studied using Cluster multispacecraft data. The structures, which have an observed duration of ~5–10 s, are approximately 200–300 km wide in the direction of propagation, and they show an average reduction in the background magnetic field of 10–20%. A majority of the events are also associated with an increase in the high‐energy high pitch angle electron flux, which indicates that the depressions are presumably generated by electrons with relatively high velocity perpendicular to the background magnetic field. Differences in the recorded electron spectra in the four spacecraft indicates a possible nongyrotropic structure. Multispacecraft measurements show that a subset of events are cylindrical, elongated along the magnetic field, and with a field‐parallel scale size of at a minimum 500 km. Other events seem to be better described as electron‐scale sheets, about 200–300 km thick. We find that no single formation mechanism can explain this variety of events observed. Instead, several processes may be operating in the plasma sheet, giving rise to similar magnetic field structures in the single‐spacecraft data, but with different 3‐D structuring. The cylindrical structures have several traits that are in agreement with the electron vortex magnetic holes observed in 2‐D particle‐in‐cell simulations of turbulent relaxation, whereas the sheets, which show nearly identical signatures in the multispacecraft data, are better explained by propagating electron solitary waves.

Two repeated low doses of doxorubicin are more effective than a single high dose against tumors overexpressing P-glycoprotein

Standard chemotherapeutic protocols, based on maximum tolerated doses, do not prevent nor overcome chemoresistance caused by the efflux transporter P-glycoprotein (Pgp). We compared the effects of two consecutive low doses versus a single high dose of doxorubicin in drug-sensitive Pgp-negative and drug-resistant Pgp-positive human and murine cancer cells. Two consecutive low doses were significantly more cytotoxic in vitro and in vivo against drug-resistant tumors, while a single high dose failed to do so. The greater efficacy of two consecutive low doses of doxorubicin could be linked to increased levels of intracellular reactive oxygen species. These levels were produced by high electron flux from complex I to complex III of the mitochondrial respiratory chain, unrelated to the synthesis of ATP. This process induced mitochondrial oxidative damage, loss of mitochondrial potential and activation of the cytochrome c/caspase 9/caspase 3 pro-apoptotic axis in drug-resistant cells. Our work shows that the “apparent” ineffectiveness of doxorubicin against drug-resistant tumors overexpressing Pgp can be overcome by changing the timing of its administration and its doses.

Another role for CO with nitrogenase? CO stimulates hydrogen evolution catalyzed by variant Azotobacter vinelandii Mo-nitrogenases.

A likely entry/exit path for nitrogenase substrates, products, and/or protons involves residues α277(Arg), α192(Ser), and α356(Gly), all of which are highly conserved among MoFe proteins from different organisms. The α192(Ser) and α277(Arg) residues form part of a hydrogen-bonded network that also involves α195(His), which interacts with a FeMo cofactor-based sulfide. The terminal amino groups of α277(Arg) are also hydrogen-bonded directly to α281(Tyr), which resides at the surface of the MoFe protein. Individual amino acid substitutions placed at position α277 or α192 resulted in a variety of effects on the catalytic and/or spectroscopic properties of the resulting variant MoFe protein. Of particular interest was the effect of CO on H2 evolution catalyzed by three MoFe protein variants, α277(Cys), α192(Asp), and α192(Glu). All three variants exhibited CO stimulation of H2 evolution under high-electron flux conditions but not under low-electron flux conditions. This observation is best explained by these variants being redox-compromised but only at the most reduced redox states of the MoFe protein. Normally, these states are accessed and operational only under high-electron flux conditions, and the effect of added CO is to prevent access to these most reduced redox states, resulting in a normal rate of catalysis. Furthermore, via correlation of the effect of pH changes on H2 evolution activity for both the wild type and the α277(Cys) MoFe protein variant under argon, with or without 10% CO present, likely pathways for the delivery of a proton to the FeMo cofactor were identified.

In-flight calibration of the Cluster PEACE sensors

Abstract. The Plasma Electron and Current Experiment (PEACE) instruments operate on all four of the Cluster spacecraft and measure the 3-D velocity distribution of electrons in the energy range from 0.59 eV to 26.4 keV during each spacecraft spin. Pitch angle distributions and moments of the velocity distribution are also produced. As the mission has progressed, the efficiency of the detectors has declined. Several factors may play a role in this decline such as exposure to radiation, high electron fluxes and spacecraft thruster firings. To account for these variations, continuous in-flight calibration work is essential. The purpose of this paper is to describe the PEACE calibration parameters, focussing in particular on those that vary over time, and to describe the methods which are used to determine their evolution.

A confirmation of the quench-cryoannealing relaxation protocol for identifying reduction states of freeze-trapped nitrogenase intermediates.

We have advanced a mechanism for nitrogenase catalysis that rests on the identification of a low-spin EPR signal (S = 1/2) trapped during turnover of a MoFe protein as the E4 state, which has accumulated four reducing equivalents as two [Fe–H–Fe] bridging hydrides. Because electrons are delivered to the MoFe protein one at a time, with the rate-limiting step being the off-rate of oxidized Fe protein, it is difficult to directly control, or know, the degree of reduction, n, of a trapped intermediate, denoted En, n = 1–8. To overcome this previously intractable problem, we introduced a quench-cryoannealing relaxation protocol for determining n of an EPR-active trapped En turnover state. The trapped “hydride” state was allowed to relax to the resting E0 state in frozen medium, which prevents additional accumulation of reducing equivalents; binding of reduced Fe protein and release of oxidized protein from the MoFe protein both are abolished in a frozen solid. Relaxation of En was monitored by periodic EPR analysis at cryogenic temperature. The protocol rests on the hypothesis that an intermediate trapped in the frozen solid can relax toward the resting state only by the release of a stable reduction product from FeMo-co. In turnover under Ar, the only product that can be released is H2, which carries two reducing equivalents. This hypothesis implicitly predicts that states that have accumulated an odd number of electrons/protons (n = 1, 3) during turnover under Ar cannot relax to E0: E3 can relax to E1, but E1 cannot relax to E0 in the frozen state. The present experiments confirm this prediction and, thus, the quench-cryoannealing protocol and our assignment of E4, the foundation of the proposed mechanism for nitrogenase catalysis. This study further gives insights into the identity of the En intermediates with high-spin EPR signals, 1b and 1c, trapped under high electron flux.

Observation and Modeling of Geostationary Orbit Electron Energization Induced by Enhanced Dayside Whistler-Mode Waves

We provide correlated observations of enhanced dayside whistler-mode waves and energetic electron acceleration collected by the CLUSTER and GOES satellites during the 23∼24 September 2001 storm. Energetic (>0.6 MeV) electron fluxes are found to increase significantly during the recovery phase and the main phase, by a factor of ∼50 higher than the prestorm level. These high electron fluxes occur when strong dayside whistler-mode waves are present. Two-dimensional (2D) numerical simulations are carried out and the results demonstrate that the dayside whistler-mode wave can contribute to such enhancements in electron flux within 24 h, consistent with the observation.

Development of High Power ECH Systems for High and ELM-like Heat Flux in GAMMA 10 Tandem Mirror

Development of high power gyrotrons and electron cyclotron heating (ECH) systems for the power modulation experiments in GAMMA 10 have been started in order to generate and control the high heat flux and to make the ELM (edge localized mode) like intermittent heat load pattern for divertor simulation studies. ECH for potential formation at the plug region (P-ECH) produces the electron flow with high energy along the magnetic filed line. From the power scaling of confining potential and electron flux in GAMMA 10, the higher ECH power generates the higher confinement potential and the higher electron flux. The peak heat-flux of more than 10 MW/m 2 has been obtained during the ECH injection within the available power of ECH. This value almost corresponds to the steady-state heat load of the divertor plate of ITER. However, we need substantial upgrade of the heating power to approach the ITER level ELM energy density. For this purpose, MW power gyrotrons at 28 GHz have been started and the development is undergoing efficiently in the ECH system upgrade program with the collaboration of Japan Atomic Energy Agency (JAEA). c

Magnetosphere–atmosphere coupling at Saturn: 1 – Response of thermosphere and ionosphere to steady state polar forcing

We present comprehensive calculations of the steady state response of Saturn’s coupled thermosphere–ionosphere to forcing by solar radiation, magnetospheric energetic electron precipitation and high latitude electric fields caused by sub-corotation of magnetospheric plasma. Significant additions to the physical processes calculated in our Saturn Thermosphere Ionosphere General Circulation Model (STIM–GCM) include the comprehensive and self-consistent treatment of neutral–ion dynamical coupling and the use of self-consistently calculated rates of plasma production from incident energetic electrons. Our simulations successfully reproduce the observed high latitude temperatures as well as the latitudinal variations of ionospheric peak electron densities that have been observed by the Cassini Radio Science Subsystem experiment (RSS). We find magnetospheric energy deposition to strongly control the flow of mass and energy in the high and mid-latitude thermosphere and thermospheric dynamics to play a crucial role in driving this flow, highlighting the importance of including dynamics in any high latitude energy balance studies on Saturn and other Gas Giants. By relating observed H3+ column emissions and temperatures to the same quantities inferred from simulated atmosphere profiles we identify a potential method of better constraining the still unknown abundance of vibrationally excited H2 which strongly affects the H3+ densities. Our calculations also suggest that local time variability in H3+ column emission flux may be largely driven by local time changes of H3+ densities rather than temperatures. By exploring the parameter space of possible high latitude electric field strengths and incident energetic electron fluxes, we determine the response of thermospheric polar temperatures to a range of these magnetospheric forcing parameters, illustrating that 10keV electron fluxes of 0.1–1.2mWm−2 in combination with electric field strengths of 80–100mVm−1 produce H3+ emissions consistent with observations. Our calculations highlight the importance of considering thermospheric temperatures as one of the constraints when examining the state of Saturn’s magnetosphere and its coupling to the upper atmosphere.

Comparison of energetic electron flux and phase space density in the magnetosheath and in the magnetosphere

Whether energetic electrons (10s of keV) in the magnetosheath can be directly transported into the magnetosphere and further energized through radial diffusion is significant in understanding the physical mechanisms for producing the radiation belt electrons (&gt;100s of keV) in the magnetosphere. In this study, we analyze more than two hundred magnetopause crossing events using the energetic electron and magnetic field measurements from Geotail and compare the flux and phase space density (PSD) of the energetic electrons on both sides of the magnetopause. It is found that for most of the events (&gt;70%), the fluxes and PSDs of energetic electrons in the magnetosheath are less than those in the magnetosphere, suggesting that the energetic electrons in the magnetosheath cannot be a direct source sufficient for the energetic electrons inside the magnetosphere. In fact, our analysis suggests a possible leakage of the energetic electrons from inside to outside the magnetopause. By investigating the average energetic electron flux distribution in the magnetosheath, we find that the energetic electron fluxes are higher near the bow shock and the magnetopause than in between. The high energetic electron flux near the bow shock can be understood as due to energization of electrons when they go through the bow shock. The relatively low flux of the energetic electrons in between indicates that it is difficult for the energetic electrons to travel from the bow shock to the magnetopause and vice versa, possibly because the energetic electrons near the bow shock and the magnetopause are all on open magnetic field lines and these two relatively intense energetic electron populations in the magnetosheath rarely get mixed.

Correlated observations of intensified whistler waves and electron acceleration around the geostationary orbit

We report correlated observations of enhanced whistler waves and energetic electron acceleration collected by multiple satellites specifically near the geostationary orbit during the 7–10 November 2004 superstorms, together with multi-site observations of ULF wave power measured on the ground. Energetic (>0.6 MeV) electron fluxes are found to increase significantly during the recovery phase, reaching a peak value by ∼100 higher than the prestorm level. In particular, such high electron flux corresponds to intensified whistler wave activities but to the weak ULF wave power. This result suggests that wave–particle interaction appears to be more important than inward radial diffusion in acceleration of outer radiation belt energetic electrons in this event, assisting to better understand the acceleration mechanism.

Evolution of energetic electron fluxes at a geostationary orbit during solar cycles 22 and 23. 27-day variations

In this paper, we consider variations in energetic electron fluxes at a geostationary orbit, which are related to the Sun’s rotation period, during two solar cycles. We use data on energetic electron fluxes obtained from the GOES (1986–2007) (>2 MeV) and LANL (1996–2007) (50–225 and 315–1500 keV) satellites. Within the solar cycle, we observed both periods of high flux peak recurrence during several rotations and periods when there was no (or a low) 27-day recurrence. We show the similarity and difference between manifestations of flux recurrences from cycle to cycle. Insignificant manifestation of recurrence and its absence were observed mainly during the solar maximum. High electron flux recurrence was observed mainly at the phase of decline in solar activity (near the solar minimum). We show that for both solar cycles, there were several days (within the Bartels rotation) during which increased energetic electron fluxes would be most likely observed.

Low-Energy Electron-Induced Decomposition and Reactions of Adsorbed Tetrakis(trifluorophosphine)platinum [Pt(PF3)4

The effect of ∼500 eV electrons on nanometer-thick films of the platinum precursor tetrakis(trifluorophosphine)platinum [Pt(PF3)4] has been studied in situ under ultra-high-vacuum (UHV) conditions using a combination of X-ray photoelectron spectroscopy (XPS), mass spectrometry (MS), and high-resolution electron energy loss spectroscopy (HREELS). Electron irradiation of adsorbed Pt(PF3)4 molecules initially proceeds through a single Pt—P bond-cleavage event and the ejection of one PF3 ligand, analogous to the electron-stimulated reactions of Pt(PF3)4 in the gas phase. The electron-stimulated deposition cross section of Pt(PF3)4, σPt(PF3)4, is governed by the rate of this initial Pt—PF3 cleavage event, which is calculated to be ∼2.5 × 10–15 cm2 at an incident electron energy of 500 eV. In contrast to the initial deposition step, subsequent electron-stimulated reactions of the surface-bound Pt(PF3)3 intermediate occur exclusively through P—F bond cleavage and the release of fluorine into the gas phase. In this second phase of the decomposition process, oxygen uptake into the film is observed because of reactions between water vapor and the coordinatively unsaturated phosphorus atoms formed by P—F bond cleavage. Electron-beam-induced deposition (EBID) of Pt(PF3)4 was also performed by electron irradiating a substrate at room temperature and at higher electron fluxes, in the presence of a constant partial pressure of Pt(PF3)4. The absence of fluorine in these films underscores the role of electron-stimulated P—F bond cleavage, whereas the absence of oxygen highlights the important role that deposition conditions (e.g., substrate temperature and background gas composition) play in determining the ultimate composition of typical EBID films.

A Large-Scale FPGA-Based Trigger and Dead-Time Free DAQ System for the Kaos Spectrometer at MAMI

The Kaos spectrometer is maintained by the A1 collaboration at the Mainz Microtron MAMI with a focus on the study of ( <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">e</i> , <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">e</i> ' <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">K</i> <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">+</sup> ) coincidence reactions. For its electron-arm two vertical planes of fiber arrays, each comprising approximately 10000 fibers, are operated close to zero degree scattering angle and in close proximity to the electron beam. A nearly dead-time free DAQ system to acquire timing and tracking information has been installed for this spectrometer arm. The signals of 144 multianode photomultipliers are collected by 96-channel front-end boards, digitized by double-threshold discriminators and the signal time is picked up by state-of-the-art F1 time-to-digital converter chips. In order to minimize background rates a sophisticated trigger logic was implemented in newly developed Vuprom modules. The trigger performs noise suppression, signal cluster finding, particle tracking and coincidence timing and can be expanded for kinematical matching ( <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">e</i> ' <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">K</i> <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">+</sup> ) coincidences. The full system was designed to process more than 4000 read-out channels and to cope with the high electron flux in the spectrometer and the high count rate requirement of the detectors. It was successfully in-beam tested at MAMI in 2009.

Front-End Electronics for the KAOS Spectrometer at MAMI

A new front-end electronics system has been developed for the electron arm tracking detectors in the Kaos spectrometer at the Mainz microtron MAMI. The signals of multi-anode photomultipliers are collected by 96-channel front-end boards, digitized by double-threshold discriminators and the signal time is picked up by F1 TDC chips. The system was designed to process more than 4,000 channels and to cope with the high electron flux in the spectrometer and the high count rate requirement of the detectors. A subset of 288 channels was installed and successfully used in the 2008 data taking period of the Kaos spectrometer.

Intermediate-term periodicities in relativistic solar electron fluences during solar cycles 22 and 23

In this paper, we have investigated the intermediate-term periodicities of the relativistic ( E > 10 MeV) solar electron flares measured by IMP-8 satellite of NASA for the time period of 1986–2001. This period of investigation includes the entire solar cycle 22; ascending, maximum and a part of descending phase of the current solar cycle 23. To determine accurately the occurrence rate of electron flux, we have employed three different spectral decomposition techniques, viz. fast Fourier transformation (FFT); maximum entropy method (MEM) and Lomb–Scargle periodogram analysis method. For solar cycle 22, in the low frequency range, power spectrum analysis exhibits statistically significant periodicities at ∼706, ∼504 and ∼392 days. In the intermediate frequency range, we have found a series of significant periodicities ∼294, ∼221, ∼153, ∼86, ∼73 and ∼66 days. For short term, periodicities of ∼21–23, ∼31 and ∼37 days were found in power spectrum. When solar cycle 23 is considered the significant periodicities are ∼20, ∼23, ∼29, ∼39, ∼54, ∼63, ∼118, ∼133 and ∼154 days. These results provide evidence that the best known Rieger period (∼153 days), appeared in the high energetic electron flux data for cycle 22 and also likely during maxima of cycle 23. The existence of these periodicities has been discussed in the light of earlier results.