Timo Bräysy

Ion cyclotron waves during a great magnetic storm observed by Freja double-probe electric field instrument

Evolution of the great magnetic storm in April 1993 is studied using observations of electromagnetic ion cyclotron (EMIC) waves by the F1 double-probe electric field instrument onboard the Freja satellite. The almost continuous operation of the F1 instrument in the overview mode allowed us to follow the global EMIC wave activity at low altitudes above the ionosphere during several subsequent days covering the initial (compression), main, and recovery phases of the storm. During the initial phase of the storm the spatial occurrence of EMIC waves has a postnoon high-latitude maximum, in agreement with earlier statistical results. A sudden and dramatic change of this pattern was observed with the start of the storm main phase. During the main phase, wave amplitudes were greatly enhanced and the active wave region moved to considerably lower latitudes to the late evening MLT sector. Also, the existence of heavy ions in the later main phase changed the distribution of wave frequencies dramatically. Most interestingly, a number of oxygen band EMIC waves were observed during a limited period of about 7 hours in the later main phase. The observed asymmetric MLT distribution of these oxygen band waves implies that the oxygen loss rate is faster than the drift rate. The results suggest that the EMIC waves play a crucial role in the main and early recovery phase of a great storm.

Pc1 pearls revisited: Structured electromagnetic ion cyclotron waves on Polar satellite and on ground

We study an event of structured electromagnetic ion cyclotron (EMIC) waves observed by the Polar satellite in good conjunction with the Finnish ground stations on April 25, 1997. Polar observed two EMIC wave bands around the plasmapause. He + band waves consisted of repetitive bursts which were observed on ground as a classical Pc1 pearl band. H + band waves occurred over a large latitude range of more than 5° invariant latitude and were observed on ground as a broad, diffuse Pc1 pearl band with several subbands. We found the same repetition period of ∼100 s for the Polar He + band bursts and ground Pc1 pearls, in conflict with the bouncing wave packet (BWP) model. Comparing the burst structure of He + band waves in Polar and on ground, we found a transit time of ∼45 s and an average group velocity of ∼500 km s -1 . Within the BWP model this velocity would lead to a pearl repetition period of more than 250 s, in dramatic contradiction with the observed repetition period. Moreover, the bursts of the two Polar bands were roughly simultaneous with no significant dispersion, contrary to the expectation of the BWP model. These results clearly reject the classical BWP model, i.e., that Pc1 pearls are generated by one wave packet bouncing from one hemisphere to another. Instead, we find that EMIC waves were accompanied by long-period ULF waves which had a period very close to the repetition period of the simultaneous EMIC bursts. Interestingly, plasma density showed simultaneous fluctuations with roughly the same period. As an alternative to the BWP model, we discuss models where the EMIC wave packet structure and ULF waves are connected. We note that the suggested relation of EMIC wave packets and ULF waves offers a new explanation to the well-known preference of Pc1 pearls for the plasmapause. We have also estimated the full Poynting flux of EMIC waves for the first time, using the three components of electric and magnetic fields. The magnitude of the total Poynting flux of the He + band waves was ∼20-25 μW m -2 and strongly directed downward away from the equator.

Dispersive Pc1 bursts observed by Freja

Electric field measurements by the Freja double probe sensor are used to study equatorially generated ion cyclotron waves, also called Pc1 pulsations. We have examined the global occurrence and spectral properties of these waves in the upper ionosphere during 12-hour period on Nov. 18, 1992, when a long chain of structured Pc1 waves (pearls) was observed on ground. In agreement with ground observations, Pc1 waves were found to occur as short bursts of 10–25 s in early morning to postnoon MLT sector. Most Pc1 activity was detected within a small latitudinal range, extending from 60° CGMlat at dawn to 63° CGMlat at noon. The latitudinal width of the source was only about 0.5° CGMlat. Observations give evidence for a plasmapause connected source region that was several hours wide in MLT and active during many hours. One burst displayed a fully developed classical dispersive Pc1 pearl, now detected for the first time above the ionosphere. In all studied Pc1 events, two spectral maxima (bands) were observed. The longer Pc1 wave bursts showed evidence for a small time delay between the lower and upper frequency bands, unveiling a new dispersive phenomenon.

Non-stationary Alfvén resonator: new results on Pc1 pearls and IPDP events

Abstract We analyse a Pc1 pearl event observed by the Finnish search-coil magnetometer network on 15 December 1984, which subsequently developed into a structured IPDP after a substorm onset. The EISCAT radar was simultaneously monitoring the mid- to high-latitude ionosphere. We have calculated the ionospheric resonator properties during the different phases of the event using EISCAT observations. Contrary to the earlier results, we find that the Pc1/IPDP (Interval of Pulsations of Diminishing Period) frequency observed on the ground corresponds to the maximum of the transmission coefficient rather than that of the reflection coefficient. This casts strong doubts on the bouncing wave packet model of Pc1 pearls. Instead, we present evidence for an alternative model of pearl formation in which long-period ULF waves modulate the Pc1 growth rate. Moreover, we propose a new model for IPDP formation, whereby the ionosphere acts as an active agent in forming the IPDP signal on the ground. The model calculations show that the ionospheric resonator properties can be modified during the event so that the resonator eigenfrequency increases according to the observed frequency increase during the IPDP phase. We suggest that the IPDP signal on the ground is a combined effect of the frequency increase in the magnetospheric wave source and the simultaneous increase of the resonator eigenfrequency. The need for such a complicated matching of the two factors explains the rarity of IPDPs on the ground despite the ubiquitous occurrence of EMIC waves in the magnetosphere and the continuous substorm cycle.

A novel approach to fair routing in wireless mesh networks

Multiradio wireless mesh network (WMN) is a feasible choice for several applications, as routers with multiple network interface cards have become cheaper. Routing in any network has a great impact on the overall network performance, thus a routing protocol or algorithm for WMN should be carefully designed taking into account the specific characteristics of the network. In addition, in wireless networks, serious unfairness can occur between users if the issue is not addressed in the network protocols or algorithms. In this paper, we are proposing a novel centralized routing algorithm, called Subscriber Aware Fair Routing in WMN (SAFARI), for multiradio WMN that assures fairness, leads to a feasible scheduling, and does not collapse the aggregate network throughput with a strict fairness criterion. We show that our protocol is feasible and practical, and exhaustive simulations show that the performance is improved compared to traditional routing algorithms.

A modulated multiband Pc1 event observed by POLAR/EFI around the plasmapause

Abstract We study an electromagnetic ion cyclotron (EMIC) wave event observed simultaneously by the Electric Field Instrument (EFI) on board the POLAR spacecraft and by the Finnish pulsation magnetometer chain on April 25, 1997, when the two were in a good conjunction. EFI recorded waves at two frequency bands from L = 4.3 (in the outer plasmasphere) to L=6.2 (just outside the plasmapause). Both bands were observed in several conjugate stations on ground. The waves showed repetitive variations in amplitude, corresponding to classical Pc1 pearls. The repetition period was the same on ground and in space. Moreover, the repetition period of Pc1 pearls coincided with the period of simultaneous Pc4 waves observed by POLAR and on ground. The observations suggest that Pc1 pearls (EMIC waves in general) are modulated by Pc4 waves rather than result from the bouncing of a wave packet from one hemisphere to another.

The EULER project: application of software defined radio in joint security operations

The task of improving the effectiveness of public safety communications has become a main priority for governments. This is partly motivated by the increased risk of natural disasters such as flooding, earthquakes, and fires, and partly due to the risks and consequent impact of terrorist attacks. This article focuses on the experience from the European Commission Seventh Framework Programme project known as EULER, which seeks to demonstrate the benefits of software defined radio technology to support the resolution of natural disasters of significant stature, which require the participation of different public safety and military organizations, potentially of different nations. In such scenarios, the presence of interoperability barriers in the disaster area is a major challenge because different organizations may use different wireless communication systems. In this context, the main aspect investigated in EULER is the definition of a common waveform that respects the software communications architecture constraints, and guarantees maximum portability across SDR platforms. This article discusses a range of issues that have been identified thus far within the EULER project; in particular, the perceived pan-European interoperability needs of public safety, and coordination with military devices and networks. Aspects of interoperability are also extended to the three dimensions of platform, waveform, and information assurance.

Analysis of cognitive radio networks with imperfect sensing

Recently, cognitive radio access has received much attention. Spectrum sensing methods are often used for finding free channels to be used by cognitive radios (secondary users). State diagram based approach can be used for analyzing the effects of imperfect spectrum sensing (with false alarms and misdetections). The state diagram consists of two-tuples like (1,2) meaning one primary user and two secondary users present. We note that state dependent transition rates are very important for accurate modeling. This is because for example in state (3,0) (all channels occupied by primary users) collisions happen with increased probability. Our contribution is as follows. Explicit expressions for state dependent transition rates are presented for the case with three channels. However, the approach can be used also for more channels. Primary termination probability is used for evaluating the level of interference to primary users caused by secondary users. Secondary success probability is used to find out how often does a secondary call start and terminate successfully. Simulation and analysis results agree very well.

Bi-code channel access method for ad hoc networks

Ad hoc networks operate conventionally on a contention based common channel where harmful collisions bring problems with increasing traffic load. The use of code division multiple access (CDMA) increases the capacity by allowing multiple successful transmissions simultaneously within the limits of multiple access interference (MAI). However, so far, most of the studies on CDMA ad hoc networking have concentrated on complex schemes like transmitter based orthogonal coding. This paper takes a cross-layered design approach for ad hoc networking, and proposes a CDMA-based bi-code channel access methods. In this method, two channels are tracked at the receiver. One is used as a dedicated receiver based channel for robust data transmission, and the other is used as a common access channel. It is shown that this method outperforms traditional methods and still is easy to implement over different platforms, robust for the design parameters and different conditions while maintaining the original idea of ad hoc networking.

On the effect of traffic models to the performance of ad hoc network

Ad hoc networks have been proposed as a topology for future tactical networks. The performance of ad hoc networks and especially ad hoc routing protocols has been studied widely. In most cases, the performance is studied with respect to mobility and network size. In this study, we will concentrate on the effect of traffic models under different traffic loads to the performance of ad hoc routed networks. In this paper we use AODV routing protocol over IEEE 802.11b MAC-layer. Research is carried out by simulations with Opnet 8.1 network simulator. Data traffic is usually bursty, which causes serious congestions. Throughput is collapsed due to large number of collisions. Another problem is increased transmission delays. These problems can be crucial especially in tactical networks. Our simulations show that the nature of multi-hop communication causes these problems to be multiplied in an ad hoc environment where significant loss of performance is observed.